I have a mac which I use only as a terminal into my Linux machine. Here are the positives:
1) No viruses (that we know of). Though perhaps this is due to the platform's lack of popularity as much as Unix security. If I were a bot writer, I wouldn't even bother with macs.
2) Sweet industrial design.
3) Unix underpinnings.
4) Though I find it annoying, the use of meta keys in preference to Ctrl is the right thing. Linux and Windows have it wrong.
5) Fast boot times.
What I don't like:
1) Despite making their own hardware and software, there are still lots of hardware problems. I don't need to enumerate them all. Just skim through the forums. Though in my case (G4 Mac Mini), the DVI output is weak. When connecting via a VGA adapter, the picture is very dark. The DVI interface fails to work with many monitors. I had to buy SwitchResX to get mine to work. The HDD has that incessant head parking problem. Worst of all, these problems are still there with new hardware almost 3 years on.
2) I'm not much of a gamer, but gaming does not appear to be the Mac's strong point. Unless you count virtualization, in which case you need to shell out $$ for a windows license.
3) I'm not convinced by the UI. Resizing windows only from one corner. Weird window management - one really does need Expose to get around. Look-and-feel is not consistent across applications (mainly with older, less popular programs, but still).
4) Shortcuts are not consistent across different programs.
5) Compatibility with 3rd-party H/W is much worse. You really have to make sure that the manufacturer has Mac drivers. With Windows it's a given.
6) Not sure about this one, since I've only ever run Tiger, but why do you need to update your application to a new version, when updating OS X? Windows can run some ancient programs without trouble.
7) If you need something, you have to pay for it. Even if it's a one-off utility.
Mouse cursor affected by some sort of screen molasses.
9) Finally, and this one just kills the Mac for me, the blurry smudgy font rendering. I know that it's the more correct method and it's WYSISWYG, but until we have 300 dpi screens, Linux and Windows are way more readable.
Perhaps if I used my Mac for more i.e. was one of those creative people in the TV ads: editing videos and photos and... running garage band, I'd have more positives. Right now I try to use mainly Linux. It's far safer than Windows, and has better UI and font rendering than OS X (IMHO).