It's an Air-like laptop with real performance. It has a real graphics card, can support 16 GB of ram and has a rockin Ivy Bridge processor. The retina display is very impressive as well.
It is in fact marginally lighter than the 13" MacBook Pro, but the difference from the 15" model is less than on the 13" Air. The 13" MacBook Pro is 1.35 kg, and the 13" Macbook Pro is 2.05 kg (53% heavier than the Air). The MacBook Pro with Retina Display (mid-2012) is 2.02 kg while the 15" MacBook Pro is 2.56 kg (27% heavier).
Its the fragility of the retina display that has me has hesitating. I'm still using an iPad 1; I decided not to upgrade after reading that the retina screen on the iPad 3 cracks on drops that have yet to put a dent in my 1st-gen. If my gf gets the new MBP, the warranty will be absolutely necessary (she drops her laptop constantly, SSD's were a godsend for her).