In my testing so far on a Windows test server (I know, I know, huge difference! I need to test on the Ubuntu and Debian boxen next) performance has dropped by 50-100% by going to 5.5. At this point, my test bed has a wide range of results but in general: MariaDB 5.1 136sec, MySQL 5.1 170sec, MySQL 5.5 305sec. The only difference between these runs is the removal of skip-locking and the running of mysql_upgrade.exe to update some tables for stored procs on 5.5.
I can't roll it back without a dump and reinstall at this point, which I have done several times.
MySQL 5.5RC is exactly that, a release candidate. That I would vote NO on for now...
Update: MySQL 5.5 on CentOS 5.2 is easily 6 times faster (28sec vs. 170sec) than MySQL 5.1 for our application. I re-ran the tests about 15-20 times, and 5.5 on Linux HAULS! 5.5 completes the same benchmark as above in 28 seconds, give or take a couple. MariaDB is still the winner on Windows unless we can figure out what the deal is with 5.5.
Hardware-wise, the Linux box had it all stacked against it: virtual, only 256MB RAM, local sata disk, completely unoptimized, and still ran away with it. The application has never been faster. Now on to replication!