17 inch user here.
The 17" is excellent.
Anyone who says its too big needs to buy a gym membership.
Seriously though, same thickness as the 13 and 15 inch, only slightly larger and heavier. I can pick the 17" umbp with one hand (gasp), put it in my bag (gasp), carry it (double gasp) AND put it on a desk, and no the desk isn't made out of some high tech super strong metal alloy.
I'm 50/50 at my desk and on the go, and it travels fine. The high res means I can have lots of windows open.
Though it is a purpose driven machine. If you have no use for the extra speed nor the expresscard slot, then go for the 15 inch which is a very fine machine. If you think the 17" is out of your league simply because it's 17", check on out in the store and decide for yourself. It may be too large for your specific purposes, but I can attest to it's portability.