Pieter fixed the "100% usage on OSX bug" and I created and uploaded a fixed version 0.6.2.2 especially for the Mac.
RE: upgrading: the best upgrade practice is to:
1) Backup your wallet someplace safe (if you're running an 0.6 release then use the Backup Wallet menu item).
2) Shutdown. Make sure Bitcoin isn't still in your system tray-- don't just close the window, shut it down.
3) Wait a minute or two (it can take older versions a minute or two to completely shut down)
4) Install and run the new version.