i bought this lens a bit over a year ago for my then-new 30D. i considered the Canon 100-400 "L", as well as the Tokina 80-400, and also the Sigma EX 50-500, but settled on this one because it was much cheaper than the Canon, and had OS which the other two did not. i now find that it is the most-used lens i have, and is mounted on my 30D 90% of the time.
i've heard the comments that it's too slow, but no one i've talked to can tell me how much slower it is than the Canon 100-400 L. what i have observed in using it for wildlife, birds in flight, etc., is that it focuses as fast as i need it to. if i'm worried that it may not lock on quickly enough, i simply give the focus ring a twist in the right direction, and the focus lock is almost instant. the AF mechanism makes a bit of noise, but it's negligible, and certainly not a problem in any setting i have used it in.
the OS is superb! i routinely use this lens handheld at 400mm for shutter speeds down to 1/125, and have even gotten good shots at 1/80 or 1/60, though that's pushing it a bit. the OS works so well, you can actually see it kick in when you half press the shutter... the image actually stabilizes and becomes rock solid before your eyes.
build quality is excellent. the lens is solid, with no rattles and none of the cheap, "plastic" feel of many lesser lenses. i also like the soft, satin-matte finish. the zoom is smooth, if a bit slower than the "trombone" style of the Canon, and manual focus has a nice feel, though it seems just a bit touchy for my eyes.
optical quality is where this lens really shines. it is sharp, contrasty, and with good clarity and color rendition. it can be a bit soft in the corners wide open, but one or two stops down and it's plenty sharp. i would put the image quality this lens produces up against the Canon "L" lens any day. and for $350 less than the Canon, at that! after all, image quality is what it's all about... and this lens, handheld at 350mm, 1/250 @ f5.6, got me a shot that was published in National Geographic. that says it all, as far as image quality is concerned.
for anyone looking for a top-quality, well built, stabilized lens in this focal range, i would highly recommend this lens, if you can find one. they are hard to come by, but worth the search!