Jumped straight from 5.5 (plain, no contribs at all) to 6.0b3 ... machine wouldn't start dnscache automatically, wouldn't serve webpages to Internet, wouldn't do email retrieval properly, etc. even after in-place upgrade to 6.0.01-01. Next step is to back up the files, wipe it out & start from scratch.
I have a 5.6 box on my home LAN that I successfully upgraded from 5.5 with MANY contribs installed, but I'm afraid to jump to 6. So as long as there aren't any critical security issues, there it stays.