Echo-ing other praise.. I wanted to write my two cents for those who are on the fence about buying this lens, not because of the question of quality, but because they're not sure if they want to spend that much money on the 1.2 versus 1.8, or comparible zoom range.
Short answer, it is worth it. I've used all common L zooms, 85mm 1.8, other primes, the 85mm 1.2 mk I, and this lens has surpassed them all when it comes down to simple image sharpness, color/contrast rendering, and build quality.
I use it on a 5d, have for approx 3 weeks for several projects and test shoots, and am one happy customer. So if you can afford it & need the 1.2 or focal length, buy it now!
I ignore comments about weight, slow-focus, and price-point, because they are missing the point about what this lens was made for.