I've owned several 15" Powerbooks and switched to the 17" when it came out. It's great, the added screen real estate is really nice to have for just about everything I do. At first it seemed kind of big on my lap, now a 15" Powerbook looks so small.
I went through the whole decision making process last summer and ended up with a 15". I really like the lightness and compact size and it's giving me plenty of screen space for my apps. I don't regret not getting the 17".
But it really is a personal decision as to how you weigh the value of portability against screen real estate.
I've got a 17" and I'll tell you a couple of things.
1. The screen is awesome
2. They are thinner than the 15" (because of the width)
3. No matter what people might say they fit perfectly on the airplane traytable (almost like they were spec'd to be the exact width of these puppies).
I love mine and wouldn't go smaller if I were you.