After purchasing this lens I had to join the chorus of praise heaped on it. I had all the intentions of getting the Canon EF-S 17-55mm f/2.8 IS to go with my new Canon EOS 60D. However the recent January-March price drop of $60 sealed the deal. I figured it was worth the gamble, or as I've often seen it referred to "the Sigma lottery". The reviews on this lens have been in for a long time, and most of them are quite positive. Some even rate this lens better than the venerable Canon EF-S 17-55. However, there is also the black cloud that seems to hang over Sigma's head - that of variable quality control. You'll see the bitter reviews of folks with bad experiences right here.If you hang around the camera forums, this can get to be even more depressing. People going through all kinds of tests for their lenses, constantly trading them back in for better copies. Or, claiming after going through 2 or three or five copies finally giving up. The brand that usually seems to pop up most often is Sigma. Perhaps because they (more than others) have a range of lenses that go toe-to-toe with the big boys. In some cases they're just as pricey as Canon or Nikkor.The 17-50 is claimed by some to be as sharp, and even sharper than the 17-55 at above f4. For almost half the price I figured it was worth a shot, and this seems to be one of those rare cases where I really feel I got my monies worth. There are some trade-offs in features, but not quality. One thing I'll mention right away that I've seen mentioned elsewhere, and that's a "rattling" noise the OS makes when you shake the lens. Now, it never occurred to me to shake my lens. But I guess I'm odd that way! So, now than I finally have the thing I shook it. You know what? I don't hear it. I mean, I can hear a bit of movement, but nothing like my EF-S 17-85 for instance. Take that for what it's worth!I decided to first test the lens on my "I know how it works and I know it's working" XTi Rebel. The first shots looked promising, and further tests showed it to be sharper than many reviews I've seen. I've posted a photo of the lens center and extreme corner at 17mm f2.8, where the lens is purported to be at its worst. It only gets better from there! After pairing to my 60D, well, keep reading!Fit and finish:This new Sigma no longer uses their matt crinkle sort of rubberized finish. While I liked the old finish, it showed marks, collected dirt and is very hard to clean. The new matt finish is smooth and nice, perhaps nicer than the speckled finish of the Canon EF-S series. The lens feels as of-a-piece, there's not a hollow spot or loose fitting anything anywhere. One complaint I see popping up regards the lens hood fit. At first I was all there with the "it barely hangs on" thing. Jeez, what's up with that? Oh. Duh. You rotate it a bit harder and it snaps in place. A firm "click". Nice. It's not going anywhere. This is very different from past Sigmas and the Canon EF-S lens hoods, which while nicely finished are still flimsy and don't fit all that great. The interior is not ribbed like old Sigmas, nor flocked like Canons, just a simple matte finish. At least it didn't cost $35! Other than that you'll find a zoom lock, AF/Manual and OS On/Off switches. One bizarre characteristic can be seen when viewing the mount and back element. If you zoom the lens out and the element travels inward, the camera's electronics are exposed! I found this weird enough to check my other lenses, and no, this is unique. I guess just don't check it out in a rainstorm and you're good to go. My final comment is that there is no distance scale window (just a distance scale on the barrel), an obvious price cutting issue. I don't really use the scale for a lens this size, but miss the little jewel like window all the same.Function:It is extremely solid and operates smoothly. The zoom is nice and smooth with a good amount of equal resistance in both directions. There is zero creep, and even still there's a zoom lock. This can't be said for the EF-S 17-55. The biggest issue I've got as others have mentioned is the overly thin rotating focus ring and short focus rotation of about 45 degrees (!) with hard stop. This is not a big issue for me in still photography, but it seems this could be a problem with video and somebody as ham-fisted as I am. The feel of the focus ring while smooth offers almost no resistance. In comparison, my other lenses are far superior in feel. Again if manual focus is a priority then this should be a consideration. The switches are a huge improvement over some past Sigmas, and each snaps quickly and firmly with identical feels. The zoom lock is quite small and there's no chance of accidentally hitting it, but it's large enough and has a positive enough feel to find and engage with confidence.Performance:The AF is fast, as fast as any lens I've got. Perhaps it's so fast because of the short focus ring distance? It's not at all noisy. No hunting, no hesitation. Even in low contrast conditions it's just bang on. Considering the horror stories I've heard about the AF, this is another one for the mystery files. OS is another thing - while I've got a pretty steady hand (at certain times of the day, depending on barometric pressure and planet alignment) I'll assume it's doing its job. All the low-light shots with slow shutter speeds I've taken look great - consistent down to 1/10. 1/6 not so much. They claim 4 stops worth so I'll assume it's 3 and call it good! The OS is not at all noisy either, it was enough to concern me at first! I had to put my ear up to it and depress the shutter to confirm its operation when I first started. Summary, AF and OS are everything they should be and more.IQ:It's incredibly sharp across the board. At 17mm and f2.8 is tack sharp center, with a slight softening in the corners - and I do mean slight. at 28mm and above it's mind-boggling. Seriously, I simply can't believe how sharp this thing is wide open. I'd have to say if there's a sweet spot it's around 5.6, but it really is wonderful up to about f/11. Typically, pixel peeping at 100% can be a depressing thing on a Canon APS-C camera. Now I just giggle!Vignetting is quite apparent at the shortest focal length as would be expected, but very evenly gradated. CA is evident in extreme corners at all ranges, most notably distant high contrast objects. On my copy it only seems to be red fringing towards the outside, highly controllable in PP. Barrel distortion is pronounced at 17mm both vertically and horizontally. In comparison, my Sigma 10-20 f4-5.6 has almost no vertical distortion at 10mm. Distortion is pretty much gone by around 24 up. Really nothing unusual for this type of lens. I've actually found my distortion problems go away after owning a lens for a few weeks anyway! Why? I tend to stop shooting brick walls and start shooting other things instead. Of course, the vignetting and distortion are easily fixed as is CA using Lightroom, Photoshop, DxO or a host of other applications.Contrast is another place this lens really shines, as what I would expect from an L series lens. Colors are deep and rich if slightly on the warm side. Flare seems well controlled, I've shot pretty much into the sun with no issues. Bokeh? Meh. It's good, pleasing actually, but nothing I'd call creamy or recommend the lens for. Bright objects do tend to have outlines, but no real fringing, odd shapes, holes or halos. You'll be able to find lots of samples both here and in reviews at lenstip dot com or photozone dot de.Pros- Great fit and finish- Excellent size and weight- Fast, accurate AF- F2.8 is better than usable!- Sharp, great contrast- Better than advertized corner sharpness- Low CA- Image Stabilization- 60% the price of the EF-S 17-55- Comes with case- Comes with lens hood- Center pinch and edge pinch lens cap like most "modern" lens manufacturers (hello Canon?)- Great warrantyCons- Skinny focus ring- Rotating focus ring- Manual focus turn only 45°- Focus ring hard stop- Mediocre focus ring feel and resistance- No full time manual focus- Vignetting at lower focal lengths- No distance scale window- Short focal length- Possible QC issues- Always run the risk of future EOS body incompatibility with third party lensesIf you need an f2.8 type lens, I highly recommend giving Sigma a look. If you're looking to replace your kit lens, or are really only going to have 1 lens you may consider something with more of a focal length like the EF-S 15-85 f3.5-5.6. Basically in this category of lenses there are only about 4 choices. The Canon EF-S 17-55, the Tamron AF 17-50mm F/2.8 XR Di-II LD SP , the Tamron AF 17-50mm F/2.8 SP XR Di II VC and this. IMHO, the Tamron lenses are out of the running for either being sharp with no VC, or not sharp with VC. Tamron build quality is good but not great IMO. The Canon is top notch IQ but not top notch build, and then (are you listening Canon?) you still have to pony up for a mandatory and potentially lens-saving hood.All I can say is this is a perfect "specialized" walk-around for crop bodies as others have mentioned. For indoor shooting it's a winner. You've got f2.8 in a small well built package that's razor sharp at f4 and above. See my chart test for opinions on f2.8. When introduced this was a pricey lens and it was hard to not consider just spending the extra $200 for an EF-S 17-55. But slowly the price has come down. At its current street price of around $650, it's a deal. Buy it, test it and if you do get a defective copy, then you've been fortunate enough to buy it from Amazon, right? Send it back and get another one, it's just that easy. And speaking of easy, this one is an easy 5 stars.