I bought my first mac back in 2003. It was a powermac G4 tower. Ever since then I have been all mac all the time.
I'm no mac snob. I like the product, but have no intention of becoming part of the 'cult of mac' were I try and convert others. I think its a superior product and that's why I stay with it. If a better product comes along form another vendor, i'll move. At this point that seems like a far ways off. So I'm sticking with mac for now. Vista was interesting, but no where near what I thought it was going to be.
If you're looking at a desktop, go with the iMac, it is quite a bargain. The tower probably will be updated sometime late this summer so if you're not in a hurry I would suggest not going down that route. The Macbooks and Macbook pros are great machines as well, although they tend to really heat up after a while (i'm using a mac book pro). I would skip the mac book air, its a first generation machine and you will be paying for that privilege.
To get good tips on when to buy your mac use this site:
http://buyersguide.macrumors.com/