I'm not going to call it "good" if you want something Mac-like, but I use jGnash 2. I like mostly that it's cross-platform and open-source. It's written in Java with a Swing GUI, so it's a bit clunky on a Mac sometimes, but it gets the job done. And, of course, it's free.
All I wanted to do was use it as a checkbook register; what I ended up doing was creating a double-entry accounting system with several Income, Expense, and Asset accounts to track my spending (and income) more accurately. It works great for this purpose--and I can even open my data on Windows if I need to.
PS - The latest version seems to have issues running on some Macs (you'll have to set your preferred Java version to 6 rather than 5, I think--I had to Google the solution to get it to run for me, if I recall, and could probably help you out as well if you have issues). Version 1 works fine, but there are some features missing (and even more clunkyness).
Someone should write a nice Cocoa GUI around this....