I've recently upgraded my servers to MySQL 5.0.15.  I like the new features, and it's stable enough to run on a production server.  I have run into a few snags along the way.

First off, a lot of the code I'm running websites on has been some poorly written SQL JOIN statements.  I've had to rework those and report them myself.  I still have a few more to do though, but that's for a project I haven't gotten fully off the ground yet, thankfully.

Second, I found a problem in my own code.  Part of the problem was that the function I was using for password hashing didn't say not to use it when I wrote the code.  It does in the documentation now.  It used to return a 16 character result.  It works differently now.  I've since switched everything over to using MySQL's MD5 function instead and it works great.  I just had to reset all the passwords for a website I run.  That sucked.