If you are spending a significant amount of time creating, adjusting and refining mysql databases this software should regularly bring a smile to your face. Yes, there are web-based (free) solutions, and other applications like CocoaMySql and YourSQL, but Navicat easily justifies its initial cost with its slick, helpful interface and extra functionality (over at least the first two: I've not spent a lot of time with phpmyadmin). Small bonus for academic users: the price is about 20% cheaper.

Four stars instead of five because the Windows version has a few nice features that aren't (yet) implemented in the Mac version (Form View, Code Completion). The prices reflect these differences to some extent (ie the Windows version is more expensive), but it'd be nice to achieve feature parity across the board.