Rooting is the best thing I ever did with my Evo. With that said, there is a reason you can't do certain things when you're not rooted...you can really bork your phone if you're not careful, but it's up to you what you want to do with it.
The process of rooting your phone by itself is not going to make your phone any different. After it's rooted (and your securities are switched off), your phone will actually boot just like it always has and won't be any different. You'll have access to root only apps that will allow you to do things that you couldn't before.
For me, I wanted to get rid of bloat because I was running out of system memory. Bye bye blockbuster, nova, peep and all the other stuff that I couldn't remove from my phone. I wasn't interested in flashing a custom ROM at all.
Flashing a rom is cool though, and IMHO a must for anyone who is rooted. There are lots to choose from and the Evo dev community is still about as active as any other phone out there, which is amazing considering it's nearly a 2 year old phone. As stated above, a ROM is basically your software. Think of it as having a PC at home. You can install any version of windows you choose. All the Evo ROMs are based on the Android OS of course, but there are different versions of the Android OS besides the version that HTC has decided to release to Evo users through over the air updates.
The kernel is the link between your system's hardware and software. The kernel is going to control your processor and the amount of voltage that it's going to use, among other things.
By all means, if you have questions after reading the dummies thread, ask away. Don't do anything to your phone that you aren't comfortable with. And this is a helpful bunch that is always quick to respond.