Some law schools are more Mac-friendly than others. A Mac is certainly fine for everything except maybe the exam software. Initially the exam software was only for PC, but they have it for Mac now. I'd wait until the summer before you start. You'll get the latest technology (in other words, won't have to upgrade during law school), and you'll know which school you'll go to and know if they fully support Macs.
I'm a Mac guy and will get a new one this summer (I only have a desktop). If there is an issue with the exam software, I have a POS PC laptop that will work.
Mac vs. PC is an endless debate. It really comes down to personal preference. I've had much better luck with Macs than PCs (fewer problems, crashes, etc.).
Macs run MS Office, so there is no problem sharing files with classmates with PCs.
I have a MacBook Pro that boots up in both OS X and Windows 7 Ultimate Edition. I probably use OS X 95% of the time, it's obviously superior and more secure. But Quicken on Windows is better and I have the flexibility to run any Windows program I might need.
To the OP, wait until summer and then decide.