A lot of the fun is that it's new and different. Also I like the idea that there's UNIX beneath the skin because I wanted to get more familiar with it. Also all the applications are 'prettier' to look at and that's fun.
Of course the machine is blindingly fast but that's just because its so much newer than the pc it replaced. It is clearly more stable than Windows XP.
But, it's not all perfect:
It does crash sometimes, in mysterious ways. On Windows, I'd be impressed (and nervous) after about 3 days of uptime. On Mac that figure is more like 10 days. But stories about it staying up 6 months at a time don't seem true.
Also I don't understand the internals nearly as well so when some app misbehaves it's more mysterious to me. No cranking up of regedit and so on. I am gradually picking up how this works, but I am a long way behind where I was on Windows.
The menu stuck to the top of the primary monitor is still ridiculous. The fact that there are so many keyboard shortcuts that you need to know to be efficient is silly. And that you have this tiny resize corner control instead of being able to use all edges to resize is also primitive.
Net net - its a lot of fun, and I am glad I did it. If I was doing different kind of work I might think different :-) But I am not in the camp that says that OS X is simply superior to Windows XP. I would say it's different, in a good way.