Got my HD2 two months back. Have tried out seven different custom ROMs, and multiple updates within each ROM. In all, I am sure I have flashed ROMs at least 20 times on my HD2.
Here is my take. It is totally worth it. You get speed bumps, extra tweaks and generally a more optimised phone. And with things like Tom's Autoinstall script, you don't have to bother with installing your apps all over again.
Yes, the downside is that you still need to manually enter your email/ facebook/ twitter logins.
So what about stability issues? Does it make the phone 'crashy'? Yes and no.
The first few days were highly frustrating for me. No matter which ROM i tried, it made the phone highly unstable. Erractic crashes that required a hard reset to recover from, phone going to sleep mode and never waking up (again, a hard reset), calls coming in but not showing up on the screen...
Like the earlier poster on this thread, i ended up with a very low opinion about these ROM 'cookers' and started wondering why they had such huge fan following.
i was about to go back to stock ROM, when i stumbled across this thead on MTTY (check it on xda forums). And that was a turning point! NOT ONE crash after using mtty.
basically, everytime you change the ROM, traces of the earlier ROM are left behind, which messes up with the newer ROM. THAT is what makes the phone unstable - not the new cooked ROM or bad coding. mtty basically wipes out all traces of the earlier ROM and so the new install is totally 'fresh' - a clean slate.
My suggestion to you is to try out a few ROMs, but be sure to use mtty before each install. If you don't like the cooked ROM scene, you can always go back to stock.
I have become a big fan of OzROM. Of course, you might like other ROMs better. Only way to find out is to play around with them and decide for yourself. Also, do check out Dutty and Artemis.
All the best... and remember to mtty!