My friend who also has an Eris has been complaining about the LAG on his phone and I told him I'd help him root. There are many amazing threads on this site with tons of helpful information, but I thought I would post a copy of the email I sent him, with links and directions to flash xtrSENSE for people who basically want their phone to look and act how they're used to... only WAY better! 
So, after receiving so much help on these forums, perhaps I can now be of service to someone new looking to root.
"OK, so I wanted to give you a helper email about rooting your phone. It's not hard, but kind of confusing if you haven't read up on this stuff like I have. So you want to search the market on your phone for "Eris Root" and I think the first one that comes up is an app called "HTC Eris 1-click Root App". Here's a link to a forum where the guy who created it posted, along with some instructions and then a bunch of people talking about how awesome it is (agreed). The download link you won't need, that's the application you downloaded from the market. Also, download an app (either one) called Astro File Manager or EStrongs File Manager for later.
OK, so once you open the 1-click Root App, read the directions/disclaimers and click the button. After that, turn off your phone and turn it back on again. I believe you should see a screen like this. You're up to step 4 on his summary there. If you DON'T see that screen, turn your phone back off, then hold the Volume Up and Power button until your phone turns on, and you should see that screen. To navigate around those menus, you use the trackball to go Up and Down and to Select things; to go "back" to the previous menu you use the Volume Down button. Also, once you click an option, it always asks you if you're sure you want to do it, and you have to click again to confirm. So you then just do steps 4-10 and then your phone is technically "rooted". By making the "Nand backup" you're basically saving your phone exactly as it is right now, so if you ever do want to come back, the phone will be in that exact state it's in when you make the backup. It sounds more complicated than it is. Also, this method by itself will not delete any of your apps/messages or anything like that. However, you do have to do a few more things to get the phone all speedy and great. This next part, where you install a new "ROM" however, will remove all of your downloaded apps, settings, and text messages. Annnnnd that is the part that makes your phone zippidy slick.
There are some programs that you can download to make this next part less painful. There is an app called "Titanium Backup" which lets you backup all of your applications and restore them after you wipe your phone. Also should say, when you install a custom ROM, it doesn't touch your SD Card (so your music, photos, videos will all still be there). There is also a program called "SMS Backup and Restore" that you can use to save your text messages (if you want to) and then restore them afterwards.
I think a good ROM that's not too different from what you're used to using (other than it being faster and totally awesome) is called xtrSENSE and the one you want to download is under the section xtrSENSE Features; under all the bullet points it says: "Download xtrSENSE4.2.1" I think the easiest way to download the file is to download it to your computer (leave it as a .zip) and then plug your phone into your computer and Mount it so you can copy files to/from the computer to the phone. Drag the file onto the ERIS drive, making sure you don't put it in any folders. You can also use Astro or EStrongs to cut and paste the .zip file onto the "root" of your SD card (long-press the file, an option screen will come up, select "Cut", then keep hitting Back until you can't go any further, and "Paste" the file there). Once you do this, you're ready to make your phone awesome. If you want to back up any apps or anything else (using Titanium Backup and Restore), you should do it before proceeding any further. The next steps will erase those things from your phone. Also, you should go through your Contacts and make sure they're saved as "Google" contacts and not "Phone" contacts. Once you setup the new ROM and sign into Gmail, these contacts will automatically show up again.
Go back to this link and go down to the bold To install a custom ROM after all of this... section and start from Step 2. The only difference is for Step 3 I think he put Vol down instead of Vol up, which is what it should be. This next part deletes your apps/messages! --> Under Step 4 you have to first "Wipe data/factory reset", and then "Wipe Dalvik-cache". Then do Steps 5, 6 and 7 and you should be good to go. And trust me, it's TOTALLY worth the time to do this.
Once you set up your phone (you have to "activate" your phone again, set up Gmail, etc. which will get back all your contacts, assuming they're "Google" contacts) you can download the Titanium program from the market and it should be able to restore all of your apps. Also, usually the first time you go to the market I believe it has a list of all the apps you had downloaded, so you can re-download them then, but if you exit the Market they will then be gone."
Welp, might be kind of long-winded but that is my contribution for the day. Hopefully I can help some poor, lost soul with a laggy Eris gain the confidence to root, just as Frisco, Scary Alien, erisuser1, doogald, and so many others on this board did for me!
So, after receiving so much help on these forums, perhaps I can now be of service to someone new looking to root. "OK, so I wanted to give you a helper email about rooting your phone. It's not hard, but kind of confusing if you haven't read up on this stuff like I have. So you want to search the market on your phone for "Eris Root" and I think the first one that comes up is an app called "HTC Eris 1-click Root App". Here's a link to a forum where the guy who created it posted, along with some instructions and then a bunch of people talking about how awesome it is (agreed). The download link you won't need, that's the application you downloaded from the market. Also, download an app (either one) called Astro File Manager or EStrongs File Manager for later.
OK, so once you open the 1-click Root App, read the directions/disclaimers and click the button. After that, turn off your phone and turn it back on again. I believe you should see a screen like this. You're up to step 4 on his summary there. If you DON'T see that screen, turn your phone back off, then hold the Volume Up and Power button until your phone turns on, and you should see that screen. To navigate around those menus, you use the trackball to go Up and Down and to Select things; to go "back" to the previous menu you use the Volume Down button. Also, once you click an option, it always asks you if you're sure you want to do it, and you have to click again to confirm. So you then just do steps 4-10 and then your phone is technically "rooted". By making the "Nand backup" you're basically saving your phone exactly as it is right now, so if you ever do want to come back, the phone will be in that exact state it's in when you make the backup. It sounds more complicated than it is. Also, this method by itself will not delete any of your apps/messages or anything like that. However, you do have to do a few more things to get the phone all speedy and great. This next part, where you install a new "ROM" however, will remove all of your downloaded apps, settings, and text messages. Annnnnd that is the part that makes your phone zippidy slick.
There are some programs that you can download to make this next part less painful. There is an app called "Titanium Backup" which lets you backup all of your applications and restore them after you wipe your phone. Also should say, when you install a custom ROM, it doesn't touch your SD Card (so your music, photos, videos will all still be there). There is also a program called "SMS Backup and Restore" that you can use to save your text messages (if you want to) and then restore them afterwards.
I think a good ROM that's not too different from what you're used to using (other than it being faster and totally awesome) is called xtrSENSE and the one you want to download is under the section xtrSENSE Features; under all the bullet points it says: "Download xtrSENSE4.2.1" I think the easiest way to download the file is to download it to your computer (leave it as a .zip) and then plug your phone into your computer and Mount it so you can copy files to/from the computer to the phone. Drag the file onto the ERIS drive, making sure you don't put it in any folders. You can also use Astro or EStrongs to cut and paste the .zip file onto the "root" of your SD card (long-press the file, an option screen will come up, select "Cut", then keep hitting Back until you can't go any further, and "Paste" the file there). Once you do this, you're ready to make your phone awesome. If you want to back up any apps or anything else (using Titanium Backup and Restore), you should do it before proceeding any further. The next steps will erase those things from your phone. Also, you should go through your Contacts and make sure they're saved as "Google" contacts and not "Phone" contacts. Once you setup the new ROM and sign into Gmail, these contacts will automatically show up again.
Go back to this link and go down to the bold To install a custom ROM after all of this... section and start from Step 2. The only difference is for Step 3 I think he put Vol down instead of Vol up, which is what it should be. This next part deletes your apps/messages! --> Under Step 4 you have to first "Wipe data/factory reset", and then "Wipe Dalvik-cache". Then do Steps 5, 6 and 7 and you should be good to go. And trust me, it's TOTALLY worth the time to do this.
Once you set up your phone (you have to "activate" your phone again, set up Gmail, etc. which will get back all your contacts, assuming they're "Google" contacts) you can download the Titanium program from the market and it should be able to restore all of your apps. Also, usually the first time you go to the market I believe it has a list of all the apps you had downloaded, so you can re-download them then, but if you exit the Market they will then be gone."
Welp, might be kind of long-winded but that is my contribution for the day. Hopefully I can help some poor, lost soul with a laggy Eris gain the confidence to root, just as Frisco, Scary Alien, erisuser1, doogald, and so many others on this board did for me!
