It is expensive, yep, but definitely worth it. Think about 4 Windows PCs in one (or 2 GNU/Linux that work without nights of terminal input and setup).
Hyper-fast booting, very responsive even while working with big images/videos and having tons of tabs or apps open, nice screen resolution (1920x1080), enough to work comfortably in Photoshop and other design software.
I got the 21.5 inches because the 27 was way too much for my room (it's more than 60cm wide). The wireless keyboard and magic mouse are incredibly useful and well designed. And it has 4 USB which is enough (keyboard and mouse don't use USB).
The only disadvantage I find is the weight of it, it is quite heavy and hard to carry from one place to another (shouldn't be often so that's fine).
Oh, and that the magic mouse hasn't got middle click (I've heard you can emulate it somehow). I also wonder how long will the keyboard and mouse batteries last, it's a pain to keep changing them every now and then.
I'm in love again!