You know, I was also worried about the lack of IS, odd focal length, etc... When I bought the lens.
However, after using it all weekend it threw those concerns out the window. I mean, on a nice day I was getting 1/2000 + shutter speeds anywhere under f/2.8... At dusk I am able to get 1/640 or better at ISO 400 or 800 wide open. Plenty fast enough to stop motion and prevent blur from shaking.
I never once (yet) had a shot where I could not get fast enough shutter speeds with this lens. f/2 is FAST. Match it up with a 7D or 5DII that has ISO capabilities usable at 3200 and 6400, it will be tough for this lens NOT to work.
I don't even think I would get the IS version if it did come out... I would not really benefit from it, nor would I want to pay the higher premium for it. Let's be honest, the if they release a 135mm f/1.8 IS, it will cost well over $1,500 to $2,000 (sadly) - The original does not lack in IQ, and you only gain 1/3 stop.
Sure, IS would be nice... but is not required for this lens IMO for most situations. Just depends on the shooter and enviroment though.
As far as the focal length goes, I have no experience with it on a FF camera body... On a crop however, the focal length is awesome. You just have to treat it as a telephoto prime and you will be set. Do not expect to use it indoors, close range (other than portraits), etc... If you buy this lens understanding to use it as a telephoto there is NO problem with the focal length. It also takes a 1.4x TC very well, giving a 189mm f/2.8 lens... Makes it that much more versatile.
But I really love this lens... If I had to give up my lens collection of all but two lenses, this would be staying. The photos I got this past weekend just blow me away everytime I take a look at them. My 70-200 was no match for it, even below f/2.8... The colors and contrast are out of this world, and the sharpness is by far the best in my collection.
The reason people sell it? I would have to guess what others have said... Being a cheaper, "spur of the moment" type of lens. For me, I use telephoto most for my shooting (planes, wildlife, coasters at amusement parks, people/candids), so the 135L fit into my needs perfectly. With the endless rave reviews of this lens I think many people buy it and try to "work" it into how they shoot, versus buying it because they NEED it or have a use for this FL. There is no reason not to like this lens, honestly. By far the best Canon glass I have used.
That is just my "extended" opinion on this lens, though 