I bought a 28-135mm used (from the original owner who is a local wedding photographer) about a year and a half ago, and since I got the 17-85mm I haven't used it much. My 28-135mm has great center sharpness, but the left side of the frame is pretty soft (on a crop sensor). I would not plan on using this lens if I upgraded to a FF camera without sending it to Canon for calibration (which could be easily justified since I only paid about $110 for it). YMMV
I would say the 18-135mm is a much more useful range, and is likely to be as sharp or sharper than the old 28-135mm design (1998). Since the STM motor is a new thing, there aren't any reviews on it yet, but I am guessing it will be just about as good as USM.