When I started my new job I was given the option of having an iMac instead of the standard issue Linux PC. They're really nice looking machines and I'd seen the impressive iSight in action so I went for it. I'm a seasoned windows user (from home), Linux user (from the PhD) and after a month getting all my work up and running on the mac I'd say I'm approaching blanket coverage of the major operating systems. Most importantly I'm now in a position to tell those people who get on their high horse about their particular OS to shut up.
Because that's what's always got on my nerves the most. Linux is a lot better than windows in many respects, but a lot worse in many others. The same goes for OS X. It depends what you're doing. Take my work for example: I'm a theoretical physicist and these days the majority of my work is numerical which means getting rather large computers to perform simulations of things rattling about, over and over and over again. All these big computers use Linux and so it's easier if your systems match. More than that though, Linux has a ton of software that is designed exactly for my needs, X-forwarding is great and its heavy use of the command line is much more efficient.
If you got into computers after Windows 95 and you don't like to fiddle under the bonnet then the chances are you won't ever use the command line. This isn't a shame in itself, it's up to you how you use your computer, but it's a shame that your choice is removed because the windows command prompt is so useless. Once you get going with a good command line interface it can make a real difference to your productivity (see for example Imagemagick).
So for work I'll (hopefully) never use Windows again. At home it's a different story. At home I use the web, edit photos and play music. Linux does all this but Windows has a better look and feel. Windows works better with my laptop's hardware (and peripharels) and there is a ton of good general software made for it. In my opinion its best bit of software is Windows Media Player. This is easily the best music player available (codec problems ruin the video playing). It's great for organising music, automatically finding artwork, creating playlists, quick search, seamless playback - the list goes on. It is infinitely better than the terrible iTunes. iTunes feels like it's actively trying to make me angry. I think it's designed to annoy you into only getting your music from the iTunes store. I'm constantly searching the net on how to get it to do this or that, WMP just does it.
Security is the top thing usually thrown at Windows. I go with the usual argument that it's just because there are more Windows PCs than anything else. Get complacent with your mac or Linux box and you'll wind up the same.
So what about the mac? Well Apple have been clever and maintained support for all things unixy (it is based on unix). This means that most of the good Linux features are supported, it does X11 so I can use most Linux programs, I can SSH into it (remote login) and so on. On top of this is has the better look and feel, it runs proprietary stuff like MS Office and it's got the best video chat facility by a mile. Apple (or maybe it's mac users themselves) always seem to be chipping away at improving the user interface. Things like exposé or Quicksilver are good examples. I'm very impressed with my iMac, it's a very neat piece of kit indeed.
Bad things about the mac are well documented. Hardware is expensive and exclusive, in many ways you're buying into a much worse monopoly than Microsoft. You won't be as compatible with everyone else (not a big deal these days), oh, and did I mention how much I hate iTunes? This shouldn't matter but Apple devotees tend to get on my nerves. I think there's a lot of fashion about owning a mac. They're always banging on the MS steal this or that. Honestly, get over yourselves. Everyone's iterating towards a better thing and Apple have reused plenty of ideas themselves (eg. spaces).
If I was buying a new laptop tomorrow I'd go for a mac. For me it covers the maximum amount of my daily stuff and I do like the interface. In general I would only recommend it if you know why you want to spend that bit more. I don't think Linux is ready to be my only OS just yet but I'll always run it on the side. The bottom line is they all do the job, they all crash (despite what people say), but most importantly of all they all run firefox so you won't really notice the difference most of the time.