You are opening a can of worms here!
Basically, if you need to run Windows software (for which there is no Mac version) you may as well get a Windows machine. You can install Windows on a Mac and dual-boot into either OS-X or Windows with BootCamp, and some Windows software may run under the Parallels emulator, but running Windows on a Mac rather defeats the object of jumping ship.
There are reported to be some issues with the latest OS-X (Leopard) which I'm not sure have been resolved. The Mac OS has always been renowned for being more stable than Windows, but people have been getting the dreaded Blue Screen of Death with Leopard. Having said that, OS-X is a Posix/Unix derived OS and as such is inherently more secure and less error-prone than Windows with its horrible registry that gets in a mess.
Unfortunately Apple's prices, especially in the UK, mean that you can't really call their products good value for money. A Windows machine of similar performance would probably cost about two-thirds as much. However, if you really like the look of them, and you don't need to run Windows, why not?
P.S. Or else get a cheap laptop and install Ubuntu Linux (free!) and you'll have something which is very stable and will not run Windows apps just as well as OS-X won't.