Safari/WebKit on a Mac is faultless. I've had one crash in about a year and a half. Safari/Webkit on Windows, however, is prone to relatively frequent crashing... and not leaving any debug info. It also doesn't play nicely if Firefox is also open and you're trying to tab between them! It's a lot more stable than it used to be, and the 3.x builds are a great improvement, though.
Consequently, I use Safari on the Mac... and Firefox/Safari combo on Windows (mainly Fx). Of course, I do tests on many other browsers for development, IE6, 7 & 8 beta; Opera; and Lynx (for textual goodness).