Was definitely worth upgrading to snow leopard from leopard. My machine is noticeably quicker and I picked up quite a bit of new storage space, but it was not without its issues on the dev side of things. The most affected area so far has been mysql.
You need to get the 64 bit version:
http://dev.mysql.com/downloads/mysql/5.1.html#macosx-dmg
I know you can use my.cnf and point to your previous versions’ data files, and then run mysql_upgrade, but that wasn’t working for me. I copied all the files from my previous install’s data directory and restarted the server. I suspect this method won’t work for all the different storage engines though.
You also need to reinstall any gems with native extensions.
The install itself, it’s worth noting, was a bit strange. It took a looong time, probably 120 minutes or so. The screen went dark toward the end. I was tempted to try restarting it because it seemed like it was hanging. It did eventually restart on its own, but when it restarted the screen was dark again. Magically after about three restarts it resolved itself.