I have a 15" MBP, it suits me fine for writing. I'm sure I'd love the extra screen size of the 17" but it seemed a tad big for me when I saw it in the store. When I'm at campus, I take my iPad every day, and my MBP when I need to do serious writing.
While I did see it on display at the Apple Store, I didn't get the high-res version w/ the matted display--I didn't see a need for it and the matted option didn't impress me. Instead, I opted for the 2.2 i7 (over the i5) and upgraded it to 8GB RAM. I was going to get an Air but I really can't see using anything less than a 15" for what I do--a 17" would be nice but the added size of the machine would be cumbersome to carry around campus. Cost didn't really factor into the decision, instead it was more about buying what I need + a little of what I want. I purchased AppleCare as well as a Magic Mouse. I also picked up a new printer which ended up coming in handy before the semester started (my Samsung laser printer died after 2.5 years....piece of junk and lousy support).
As an anecdote, I know some ppl use the printer rebate to get a new printer when buying their new Mac--I've always had great luck with Epson, they seem to make the best inkjet printers. I'm using my first HP Deskjet at the moment and I've been really happy with the wireless printing. I plan to get another Laser sometime in the next few weeks, it'll either be a Brother (good printers) or an HP. The only reason I didn't get an Epson this time around was they were out of the model I wanted, the HP was free so I went with that.
Anyway, I've had the 15" MBP for about three months and it's a dream for writing.... I normally have several windows open (Pages, Endnote, iTunes, Safari, Mail, lots of Stickies, Preview, etc.) and find that Mission Control (in Lion) handles them well.
