I'm relatively new to amateur photography, and bought this lens after TRAWLING through (i think) every single review available. Three months down the line, and I'm unbelievably glad I spent the time I did researching a lens that I wanted as a decent 'walkaround'.
Focus lock is very quick and accurate - only a couple of times photographing very similar tones in low light has the lens lens suffered, but any lens in the same situation would do the same. The lens actually focuses a second time after the initial lock to get very sharp pictures. It also features FTM (Full Time Manual) focusing which I've found particularly useful.
The lens is certainly heavier than the standard 18-55 EF-S kit lens, but more than light enough to carry around all day. The construction feels very solid - the lens barrel is plastic, while the metal mount gives the lens a hugely firm and professional feel.
I've found one of the two great things about this lens is the Image Stabilisation (IS). I've used the lens a few times to photograph bands where flash photography is banned (or frowned upon), and it's come up trumps. Anywhere in even slightly low light and it has proven invaluable in keeping my handheld images sharp. Go into a camera shop, find any Canon lens with IS and take comparison shots with it turned on and off. Honestly; you'll see what I mean.
The other is the focal length. 28mm is more than handy for landscape shots (in any case you can stitch together the shots on your computer after), and having the zoom go through to 135mm is perfect for portraits and places where you need that extra reach. I think it's best suited as a walkaround lens - especially when taken travelling.
One of the downsides I've found to the lens, though, is the lack of lock on the lens barrel itself. When the camera is pointing in a downward position, the lens can creep which is frustrating. Especially annoying when I have the lens on my tripod pointing to the floor. Grr.
Other than that, I can really see nothing to complain about the 28-135 IS USM. The IS is a dream; the images are sharper than I think the price tag warrants; the USM is silent as a mouse; focusing is quick; the focal range is excellent (but then of course that depends on what you want the lens for!!); and the variable aperture range (f/3.5-5.6) is decent enough for the majority of uses.
I'm not a professional, so distortion and vignetting etc are beyond me. There are some very good review of this lens splattered all over the Web for that kind of detail.
I'm fantastically happy with this lens, and firmly believe it'll always be a part of my camera bag as my years and experience roll on! Happy buying...