I much prefer the 13'' form factor. It does seem like the perfect package, but I suppose I am biased to my use case. I need CPU power, but not a ton of graphics power. I am a college student, so being light, fitting in my backpack easily, and ***huge*** fitting well on lecture hall fold away tables is important to me. I have some friends with a 15" and they often have to support it some other way or have it sit on the edge of the lecture tables because they are not big enough to hold the whole thing up.
With that said, I'm sure for some people, the 15 is the perfect form factor for them...