The biggest two disadvantages to the 17 inch model are:
1) Finding a computer case that will fit it that doesn't suck or cost almost as much as the annual gross income of some small nations.
2) The space between the front of the computer and the keyboard.
For me, #1 is solvable. #2 is a real pain, and the only reason why I seriously considered the 15 inch at all. For someone that wears a watch with a metal band, I often feel that I should remove the watch before starting to type on the laptop keyboard, lest I scratch it all up.
But given that I don't care for the track pad/single button, and that so far I've only mostly used external keyboards, either a Dell at the office or an older Apple one at home, this really hasn't been an issue.
The huge advantage is the resolution (which really was a show stopper, as my 3 year old Dell had 1920x1200, and I wasn't downgrading) and the size of the screen to take advantage of it. I'd recommend getting the 17 inch, unless you just can't deal with the keyboard, or need the lower resolution.
-jt2