Well, I use MacOSX with Aqua on one of my personal computers (an iBook SE), and I haven't had many usability issues with it. Now, I'm not a power Mac user, so there may well be things that I don't see, but the only serious design issue I have is that many of the widgets are too big. I've got some minor issues with window layering, but it's still about 100 times better than the Windows, UI, IMHO, at least in that respect.

I'd be interested to hear your issues (and I'm not saying they're illegitimate, just that I haven't encountered them or they don't bother me). PM me if you have the time and feel like venting. If not, that's fine, too.