Bought this lens a few days ago, so not much experience with it yet (will update later), but I must say that I am favorably impressed.
Having read many reviews of this lens, I must admit that I was a bit concerned that I may get a 'dud' that is plagued by front/back focusing or that this simply would not be what I expected. Not the case at all... my copy is a real beauty! No focusing issues of any kind.
I won't go into tremendous technical detail as this is not my fortay, but I will say this... the pictures that I took in my backyard last night are super! I did compare it to my Canon 'nifty-fifty' and the results were virtually indistinguishable (and I was pixel peeping).
The Sigma motor is quite a bit quieter than the 50, that's for sure. This isn't saying much I suppose, but it isn't like the Sigma is grinding around either - compared to my other USM lenses, is it louder? Clearly 'yes' - but the USM is almost silent!
I just can't imagine that it would be a big issue for anyone in most situations. Now, would I try to a snap a pic of John McEnroe (McNasty) at the height of his serve from my courtside seat at Wimbledon in 1981... perhaps not. But then again how many of us operate under these limitations and even if we do, how often...
After significant searching, pondering and agonizing about all sorts of potential lenses, I ended up with this lens... and I am ecstatic. As with any lens choice, there is always a series of tradeoffs that must be made... budget... brightness... focal length... build quality... manufacturer (lens snobs!) to mention only a few. I can tell you, I think my decision was a good one (for me). Here's why:
Focal Range: 17-70mm (~28-112mm on 1.6x crop) covers much of what most people use most of the time. If you have more specific needs (super wide/extra long), this may require further consideration, but for me, 90% of the time this does the trick - the other 10% for me is long range soccer photos of the kiddies. When I compared other lenses that reach only to 50mm, the additional 20mm (actually ~30mm on 1.6x crop)made a difference. Here's why... for someone like me that is looking to travel light (2 lenses) this bridges all or most of the gap between lens #1 and longer telephotos that tend to start at 70 or 100 and move out to 300 or so. If lens #1 only takes you out out to 50mm, the gap can be a bit too wide - there's a lot of territory between 50 and 100 (I have a Canon 100-300). That 50mm gap turns into 80mm on crop... that's not a gap... that's a chasm!! Some may say, just move your feet... I say "no thanks, I'll just zoom".
Price: How can you beat $300 for a good quality lens that performs as well as this one does? Not saying anything about quality, but compare price to Canon 17-55 f/2.8, Canon 17-40L, or even a Tamron 17-50 f/2.8. Can't beat this deal!
Bright: True, the max aperature decreases when zoomed, but having the 2.8 for indoors is nice. Would like it to cover the whole range, but that's one of the tradeoffs that we all know and (don't) love. Here's where you can use the 'move your feet arguement to your favor - move your feet, get some additional max aperature. Max aperature at 35mm is a very usable f/3.5.
Macro: I'm not a huge macro guy, but this lens may change all of that! The macro capabilities of this lens is pretty amazing. Nice to have it in you arsenal when you are hiking down a path (or in your backyard) and something catches your eye... not essential for me, but it is really is nice to have the capability. The beauty of macro photography is that your lot line doesn't need to abutt Grand Teton National Park to get out and take photos... you can have a good time in any back yard... that's what it is all about isn't it.
So in closing, I hope you enjoyed my review and perhaps have gained some additional insight into the uses and benefits of this truly wonderful lens.
Now let's all get out there and enjoy our hobby... make it a good day!