What about mySQL 5.1.x vs 5.5.x (I'm on 5.1.70)? I hear on one side that 5.5 is slightly faster, but only if you configure it properly (not leave it at the default install)? I read that tweaking 5.5 for multicore processors is only where it will perform better?
5.5 > 5.1 every time. No real config tweaks are needed going from one to the other. The tweaks that are important all surround the amount of available resources and the types of queries that your sites run.
If you're still on 5.1 I'd definitely advise upgrading.