I'm new to this
Totally depends on how you do it. If you just start adding all the various pieces that have been ripped from 2.1, they yes you will end up with a buggy phone.
If on the other hand, you root the phone properly and install either Sholes are Bugless Beast 7.2 (there may be other, but those are the ones I've used), you will end up a phone that is less buggy than stock and tons faster. These roms include most if not all of the cool 2.1 stuff. I know this because there's a lot of us that have done it.