Just thought I'd put in my two cents as well...
I use Windows XP (Professional) at work on Dell hardware because I have no option of using a Mac, basically, and I know I will have my pro-Mac bias but I try to keep an open mind about XP.
I've had XP lock up on me a few times (needing to physically switch off the computer), but in general it has been very stable. With OS X running on a QuickSilver G4, I have never had a kernel panic, but there have probably been a similar number (i.e. rare) of occurrences over a similar period of time where it seems to have not been responding and I rebooted in the end. However, I'd expect in those instances that if I had another computer connected with through ssh, I would have been able to kill offending processes and kick start OS X again without having to reboot (?).
Windows XP is not that bad for me, but I greatly prefer OS X. The Unix core really makes a big difference to me and opens up all sorts of possibilities to users, such as the installation of Unix/X11 programs. I often wondered about the relative arguments involved in the old BeOS vs. NeXT debate but, without wanting to start a different topic, I think opening up the Mac to the Unix world and all its existing software was actually a major bonus for Mac users. In my case, this makes OS X much more "usable," particularly as I can bring work home from Unix workstations in the office and communicate with them easily from home too.
In terms of the GUI, I do prefer Aqua, but that is perhaps a matter of opinion. I do not find the changes to the taskbar in XP (eg grouping of windows) to be a great improvement. One example is being able to move items around in the Dock is a good idea, which is something you can't do with the Windows XP taskbar to my knowledge. The iApps are also very good and I certainly use iTunes very frequently.
I'm also slightly perplexed when some people (not necessarily on this forum!) say OS X inhibits the user and XP doesn't. For instance, I am the administrator for this XP machine and when I first started it up, it hid the hard drive from me and would only allow access to "My Documents!" I also find the numerous assistants rather intrusive, though I can systematically start switching things off. To me, Aqua feels more friendly, while Luna feels a bit patronizing.
We will all have different definitions of usability and will have different opinions on what is important, so this is a difficult thing to nail down. In short, my opinion is that the Unix core really empowers Mac users and that Aqua is, overall, a more pleasant experience than Luna. Anyway, that's enough rambling for now, as I've been up early this morning and my brain has not yet started working! More factual/useful posts may follow some other time!