I got my Eris a week ago, have the OTA update, and have spent dozens of hours on forums and with friends' Android phones (particularly my friend's HTC Hero). These are my findings as far as stability problems with my Eris. I cannot speak for those who are having problems with NO extra apps installed. My Eris seems to do just fine in this scenario. That said...
1) Handcent SMS does slow the phone down, and some parts of it tend to not work very right, both in coordination with 'Auto Backlight', and when other apps are running.
-In the Settings, there is a 'blur background' setting that will, once in a while, make the phone freeze (I can repeat this process).
-In the Settings, there is a Pop-Up option. There is a lot of potential for freezing when the Pop-Up option is allowed to pop up and light up the screen when the Lock Screen is on. Simply disabling this will solve a lot of peoples' freezing issues (this has been the virtual entirety of my involuntary freezes).
2) TasKill, or another task managing application is necessary, especially if you don't reboot your phone every day/few days. These phones are computers. The 'phone' applications are simply small parts of that software that make the phone work. Computers need to be rebooted, and at the very least, even with such a lean operating system, tasks need to be managed.
-I use TasKill to auto-kill when the screen is OFF. It's in the Settings. Use an 'Ignore' list to leave on programs that you want to stay on. Some things like HTC Sense, TasKill itself, and I leave com.htc.provider on 'ignore' too.
-If things are slowing down, or if you have, say, <20M of memory left at the moment, once in a while a 'Kill All Tasks' is helpful. Open up TasKill, and click the red icon at the top (or use the Task Kill Widget, like I do)
-Even if you don't Auto-Kill, or don't pay attention to how much memory is available (I use the 'Task Bar' widget installed with TasKill), go ahead and open up TasKill and Kill All Tasks once per day. They will start back up. Even the important ones will be just fine. But your free memory will shoot up to about 90M, and only the programs your phone needs will start up, until you start them all fresh.
Remember guys, everyone and their brother is writing these programs (since programming in Java isn't that tough), and nobody is perfect.
To maintain best stability:
-Try to download apps one at a time (and play with them for a bit, in the midst of normal phone use, to make sure they play nice with everything else),
-Manually turn your screen off with the red button, instead of automatically (sometimes when things happen, like a Handcent SMS event, right as the screen is about to dim, your phone will freeze)
-Kill All Tasks or reboot regularly. More if you use your phone more. (I kill all tasks when screen is off, and have 0 stability issues, with OTA update)
I'm very attached to a) my Auto Backlight, and b) Handcent SMS compared to the stock Messaging client. It's way too fully featured, the pop-ups are hard to beat, and the customizations are necessary for my privacy purposes (having a girlfriend...) But Handcent tries to do things with the auto dimming and auto screen-on that Auto Backlight doesn't seem to like. So turn off the features for your phone, or turn it off for Handcent. If you want stability, you have to choose!
I hope any of this helps someone. I've spent a lot of time figuring this out to the best of my ability. I've had to take my own battery out four times already to reset involuntarily, and a 15+ times making it freeze on purpose by setting all these settings. Good luck!
