I've taken several thousand pictures with this lens before coming to any firm-ish commentary, any firm-ish decisions.
The build quality is good, but not outstanding. The focusing ring has a disconnected feeling that is typical of anything else in the EF line of lenses- when compared to well dampened lenses. It doesn't squeak while focusing, unlike my 35mm f/1.4.
Optically it's good. In comparison to the 50mm f/1.4 it has much less veil flare which makes pictures taken at f/1.2. Images taken wide open have better detail and acutance; images appear sharper and with more detail. The edges are a bit loose, though, and coma is pretty obvious. All these cure themselves once the lens is stopped down a touch. The lens is good for images wide-open, when it hits focus.
Focus? On a 5D at near distances (think: across the table of a restaurant in low light!) the <i>system</i> becomes rather unreliable. Some pictures are sharp, some aren't- with a back-focus quite obvious. Easily repeatable. Canon, on the other hand, offers no solution, and denies anyone has ever called with the same complaint.
Focus (or mis-focus) is quite quick, and not notably different from the f/1.4.
Finally- just a word about the price. It's an expensive lens. You'll know if you need it by the aperture- if you need another 1/2 stop wide open, the only choice is this lens. That's really the bottom line. Canon isn't known for making outstanding "standard lenses" (their 50mm f/1.4 constantly tests worse than other Large Manufacturers, with corner sharpness and veil flare worse than most), and this one is only a step above that. Sadly, the focusing issue makes this lens even harder to rationalize if one likes to take pictures at close range and wide open, exactly where this lens is supposed to be used.
For $800 it is a great lens, a good value (though it would take $500 to be as great a value as the 50mm f/1.8!). For $1,000, less so. For $1,200 it's becoming quite hard to rationalize, and for $1,600, it's worse yet. Remember, for all that money, one is also buying a pixel or two of purple fringing, too. It's obvious on speculars, and it DOESN'T get better stopping down.
Time will tell if this lens is a classic. It doesn't have the immediate appeal of the 35mm f/1.4, which isn't really optically better but has a nicer (generally) drawing. It doesn't quite get to 85mm f/1.2 level either. It's a funky lens with a too-wide barrel in between Great Lenses.
I'll keep the lens. I like it more that the 50mm f/1.4: I like the flare qualities (including the greatly reduced veil), the included hood, the contrast, and the drawing. The lens is indeed a step above the f/1.4 for me, though the step is quite pricey.
---Canon review ends here---
Leica fans: This is no Summilux/Noctilux beater. Think: First version of the Summilux 50mm and you'll have an idea of what's going on here, except in a huge and bulky package that doesn't reliably focus closely on some bodies. I had better luck with the MP and Noc. Always.