What can I say about this lens that isn't said on the Internet. It's sharp - very sharp, it's bright - very bright, and for an 'L' lens, it quite a bargain.
When I took delivery, I fired off a few test shots; when the first picture lit up on my LCD panel I knew this was a terrific lens. I spent 20 minutes micro-focusing the lens, it was back-focusing 6 clicks on my 5DmkII which made a difference when you zoom right into a photo but undetectable otherwise.
The depth of field with the f/2.0 aperture can be made razor thin, and the out of focus background (or bokeh) is creamy smooth. But stop it right down, the image remains sharp and no detectable chromatic aberration.
The construction is 'L' series quality - not weather sealed, but you know you are holding something of extremely high quality when it's in your hand; but It's not too heavy: some 'L' lenses are quite weighty but this is no more that a non-L zoom.
This lens works wonderfully with the Canon 1.4x extender, with no noticeable drop-off in quality. This takes it to almost 300mm if you do not have a full frame CCD with only the loss of 1 stop.
The thing you need to ask yourself about buying this (and any prime really) is what will I use it for? If you don't have full frame, then it's obviously going to be a medium telephoto with the 1.4x added which has a lot of uses from Airshows to Zoos. A prime 135mm on a full-frame camera is a but more tricky. Candid outdoor portraits is one perfect use, 135mm takes you outside of your subject's 'personal space' so you get much more relaxed and natural results. Indoors it's tricky because you would need a very big room. It's not much good for macro photography and 135mm is too long for landscape and too short for nature. These are generalizations though. I like to take a single prime lens out with me for walks, this makes you really think about composition and your own position in relation to the subject. Something that zoom lens users don't do and it can dramatically improve your photography. With the 135mm you have to think that bit harder, you need to walk into the middle of a field to get that tree framed how you want it, you need to stand back from that interesting mushroom, you need to stalk a bit closer to that bird a little way ahead. But the bottom-line for any lens is, does it take great photos? Yes, the Canon EF 135mm f/2.0L USM takes stunning photos.