Apple's are not expensive any more - the MacMini is £399; and the all in one iMac is £679.00.
Their quality is excellent (unless you're a bit too much of an early adopter like me!). Don't buy any Apple product until it's on at least its second edition.
As for benefits - well it depends what you're doing I suppose. Ease of use and intuitiveness is impressive.
I do a lot of presentations - and my macbook pro just plugs in and just works with all sorts of TVs in meeting rooms, projectors, and monitors; I'm always seeing others with PCs fiddling with their resolution settings etc. before anything happens. Wake from sleep is instant too.
Network settings "just work" once you've set up for a few different networks - home, office, mobile, wireless, wired etc. your mac will automatically choose the best option.
OS X is a UNIX-based operating system. You can fire up a terminal and start using your local machine just as if its a unix box.
OSX comes with PHP, Apache - and you can install MySQL so its great for local development.
You can now run Windows on Mac hardware too, using an Apple released piece of software called Boot Camp.
Software wise - you get OS useful stuff way ahead of the windows world - Spotlight for example is a really fast full text search of everything on your computer - fully integrated into the OS. I can also hit a key-combination to tile either all the windows of the current application, or all open windows - then point and click to bring the one I want to the front. This kind of stuff is coming to windows in Vista - but from what I can see its not up to what Macs have had for a while.
Colour management is another area where macs have been traditionally ahead of other operating systems. There's also lots of built in MIDI stuff.
I've been using Macs personally for around ten years (three machines a Quadra 630, (A period without a personal machine), iBook, and now a MacBookPro. I've used all sorts of other machines from Sun Solaris, all flavours of Windows, other Macs. My house's server is Ubuntu. The free unixes are great for that kind of thing - but I wouldn't want one to actually work on. Irritating though it is - its essential when communicating with the rest of the world to occasionally read/write/edit a MsWord, Excel, or Photoshop document - and though you can get free unix applications to do that - none are really reliable - and you can get the applications themselves for OSX - and they have complete compatibility with those running on a PC.
My only most minor gripe is that Macs unlike the rest of the planet still use a carriage return as a new line indicator - which is just one more thing to think about.
