I've been running on Mac for about eight years now, and will only touch a PC at work because I have no choice.
As far as writing goes, MS Word is even better on Mac than it is for PC.
Mac also has the iWork Pages program, which took over for the old Appleworks word processing program. Pages still feels a bit rough, and can't seem to make up its mind whether or not it's a word processor or page layout program. The biggest problem I've found with it is that I keep thinking "how would I do it in MSWord" instead of "what makes sense to do?"
Another program,
Scrivener, is only available on Mac, and it's what I use for most of my preliminary writing.
I heartily recommend you either hit up the Apple website and take a look at the Pages demos and walkthroughs, or -- even better -- if you live near an Apple store, head in and get your fingertips on one of the machines.
You may find that the iBook/MacBook might be a bit on the pricey side, but the machines
last. I got a Powerbook back in mid 2000 and only replaced it a couple years ago. And at the time, it was the "midline" model. So if you do go with a Mac, it's money very well spent.