you'll find all sorts of opinions, but personally i am addicted to flashing roms. we have some phenomenal developers on here who have given us some amazing roms, with some great features. you will see it repeated on here time and time again - every rom runs differently on each individual phone. if you're rooted, why not try them out, and find if one isn't right for you?! if you don't find one that you like, you can always just stick with stock. choices - that's the beauty of being rooted!!
to flash a rom, here is the method i follow, and it has worked countless times. as an aside, i always recommend dl'ing a rom thru your pc, and then putting it on your sdcard, as opposed to using Rom Mgr. in my experience (and many others on here), RM can be a little glitchy. it has some great uses, but installing a completely new OS is something that needs to go glitch-free:
1. backup your apps with TiBu (or thru Astro)
2. create a backup/nandroid of your current rom (so you have something to fall back on if you experience any serious problems flashing a new rom)
3. boot into recovery
4. wipe data/factory reset
5. wipe cache
6. go into partitions menu, and format boot and format system
7. install rom zip from sdcard
8. reboot
IMPORTANT - after you sign into Google, and it syncs your phone, don't touch your phone until the market completely downloads all your apps!
9. boot into recovery
10. flash theme (if you desire something other than stock)
11. reboot
12. boot into recovery
13. flash kernel
14. reboot
15. enjoy!
