That's a tall order to fill but here's my input:
1. Sense2.5 works great as far as stability but
you should not flash any of NRG's 2.5 ROMs without using HTCRie to remove some stuff. Our Diamond just doesn't have the space given all that NRG has packed into them (not really his fault since he cooks for devices that have tons more space than the Diamond). Search this thread for postings on what to remove. The process only take a few minutes once you know how to do it. There are several known problems with NRG's Sense2.5 ROMs:
a. Ringtones that are .wma or .mp3 don't work. Only .wav work. This is an easy fix. Just convert to .wav - I posted about this awhile back.
b. WiFi doesn't work. I can't get it to work no matter what I've tried.
c. YouTube doesn't work (I didn't put enough effort to see if there's a fix)
d. Some people report SMS problems when replying via the Manila screen. I saw this myself but when I reflashed, it didn't happen again.
2. If you don't want to do #1, download the 2.1 ROMs. They're pretty good and everything works well
afaik. But I'm used to the 2.5 so I deal with the problems.
3. There's also the difference between 218xx and 235xx. I prefer the 218xx which has the normal Start icon at the top. 235xx is a little more touch-friendly and gets rid of the Start icon. It probably works better on WVGA.
Hope that helps.