linux as an OS is far more superior than windows and osx, with or without the eyecandy. consider the fact that all the applications are free, and there are tons of them, that do pretty much everything you could do on windows/mac, and more. there is a learning curve of course. linux has come a long way as an OS. hardware driver support is much much better than it was years ago. and oh the best thing is you don't ever (almost) have to reboot after installing an update or software or a driver etc. and 99% no crashes or lockups (unless messing with beta/unstable applications), and it also uses less computer resources than windows. Ubuntu is probably one of the easiest and more user/newbie friendly distributions out there right now.
justanotheradikt: basically when you attempt to install ubuntu (and many other distributions), it detects your harddrive setup, and will see that you have windows setup on it. you could either get rid of everything and start fresh, or ubuntu can utilize whatever free space you allow it to use to setup, and still maintain windows, and when you boot up you'll get a menu to ask you which OS you want to load (this is referred to as dual-booting). You can download and burn the ISO image onto a CD, and when you boot it up it'll put you straight into Ubuntu (this is called a live cd). this way you can test ubuntu, mess around. don't expect it to run THAT fast because it's running off of the CD. the best thing about Ubuntu is its forums and the fact that pretty much anything you want to do or have an issue with, is documented somewhere in the forums. that is what an open source community is all about