I started using a Mac for work last year. At home I have a PC with Windows, and have almost always used Windows OS. Usability for Mac is much better than Windows, but there is less clarity about how everything works. I guess that's the idea, though. Macs are superior for programmers, IMHO, but I guess in the ideal world we'd all be on Ubuntu

. Excel in Mac is my only hang-up; many of the deficiencies have already been noted here. Thus I'm considering a Thinkpad or Macbook Air/Pro. Gotta have something with a keyboard with all the typing that you'll do in Bschool, and Excel is THAT important to make me consider a PC. I'll wait until I pick a school to buy, though, since they typically have some standard software packages and such that they like to install.
My SO is in school right now and she has an old Mac, but my SO's not a computer/technology person. It's enough to get you by.