I stuck with 4.0.1(jailbreakme.com) since last fall, until I upgraded to 4.2.1(greenpois0n) about a week ago. I've had zero problems with greenpois0n and my phone seems to run as fast and smooth as it ever has.
I did have some problems un-jailbreaking/upgrading and ended up spending several hours to finally get the restore to work, but that was because of dumb stuff I did. Once I was upgraded to 4.2.1 the greenpois0n jailbreak was quick and easy.
I did use AptBackup to backup all my jail-broken apps and packages before I upgraded and again to restore them all afterwards. I'm pretty sure this was a giant timesaver.
For me the HDR camera upgrade and now being able to customize the boot logo worth the hassle of upgrading.