After reading enough of the anti-50L hype on the forums I decided to pick up my own copy, albeit used - for a good price (that I couldn't resist). It's a UV07xx date code and the previous owner just had it calibrated at Canon (in January 2008) so I can't speak of its performance out-of-the-box...
My impressions are that I have absolutely no problems or reservations using this lens. I purposely went out to test for issues in real-world shooting and found none. From near to far, at all apertures - this lens produces some of the sharpest, contrastiest and colorful images and ranks right up there with the other L primes, IMO. Shooting at or near wide open gives excellent DoF/bokeh effects like the 35/85L and stopped down the sharpess and detail throughout the frame is amazing. Most images need little or no USM.
I try to buy primes that offer me something tangible over the zooms, typically bright apertures and sharpness. This lens delivers on both counts - plus offers something few of the other standard L primes (currently) offer; weathersealing. To be able to go out in just about any condition - day or night, rain or shine and have no worries is great.
I wholeheartedly recommend this lens if you enjoy the likes of the 24L, 35L, 85L or 135L. It's right up there with them, IMO.