First post. (I've been reading the forum for a few weeks in anticipation of my switch to a Mac). I took the plunge and got the 17". When you have the MBPs in a side-by-side comparison in front of you, there really is no comparison. The 17 blows the 15 out of the water. The 17 also looks better aesthetically -- because of its wider footprint, it looks slimmer compared to the squat looking 15.
Here's the backstory. Prior to making the switch I found myself undecided between an MBP and an MBA (power or portability). Once I actually had both in front of me, I found myself underwhelmed by the MBA, and overwhelmed by the 17" MBP. By contrast, the 15" MBP was so-so by comparison because it wasn't that much bigger than the MBA -- and I would gladly sacrifice some screenspace for the added portability of the MBA if forced to choose between the 15" MBP and the MBA. In short, the 17 made my choice an easy one.
A last point. Given the probable September-revision of the MBPs, many here have counseled that we wait before we buy. Unfortunately for those of us who need it before the school year starts, it's a tough wait. Still, I was leaning towards waiting it out. Seeing the 17 immediately persuaded me otherwise. (That's how much wow-factor it had.) I'm aware that the next revision might top the architectural aesthetics of the current-gen ones, even if it's difficult to see how much more it might be improved such as to justify the wait and hassle. Suffice to say that I won't be losing sleep over it . . .