Today, I visited an Apple distributor nearby, armed (cough) with a good muscle memory of how my 13" feels. Actually, both the new 15" and 13" are quite close. I played around for a while with the 15". Going to the 13" afterwards, it felt way too small. What also makes a difference is that the 15"'s default scale mode is 1680x1050 whereas my 13" defaults to 1280x800. That's a lot more usable space.
As for weight and size: I wouldn't call the 15" unbalanced when I was holding it open by a corner in one hand. The fact that it's thinner makes it great to hold. So I've overcome my doubts about the size of the machine. When I order my next one, it'll be a 15".
NB IIRC I think that the default res on the MBP15 is 1440x900 (as the screen is 2880x1800), but yes - once you look at the 15 inch screen then it really does offer a lot more usable area than the 13" (when both are at their default res).