The title of my review pretty much sums it up. The two major features of this lens are its constant aperture of f2.8 at all focal lengths, and the new VC (anti-shake) feature. The previous version of this lens was superb, but lacked stabilization, so the VC is a welcome feature.
It's a very well built lens with a metal mount. Its not pro-quality, but definitely better than any kit lens I've owned. I'd give it a solid 4 out of 5 stars for build quality and design. It's quite a bit thicker than the previous version, but not much heavier. It looks huge on my D80, but its not too heavy so it works well and balances perfect on a D300.
The vibration compensation works very well. It works just as effective as any Nikon VR lens I've owned. No complaints on the effectiveness of the VC system. Some people reported it as loud when the VC motor kicks in, but I found it to be acceptable. Not as quiet as the Nikon VR2 system, but definitely not "loud" either. Others have reported that the VC doesnt kick in as fast as Nikon VR2, and its true (very very slightly) but again, not a problem. You'll be taking your time to take a picture whenever your using VC anyhow, so it really doesn't affect real-world performance.
The lens is very good. It's not quite as good as the previous version wide open at f2.8, but the previous version was so good that you can lose a bit of sharpness and still get fantastic results. With that said, wide open at f2.8 I find it very good for my type of pictures, which are generally of people. If you're into photographing brick walls or textbooks, you may be disappointed. If you're concerned more with getting great pictures, you will be pleased. From f4 on, there isn't much to complain about except the outer most part of the picture. You have to be at f5.6 or f8 to get good sharpness throughout the frame including the corners. If you need sharpness in corners as well as the center, like when photographing architecture or landscapes, you'll typically be at smaller apertures anyhow, so it may not be an issue for some. Distortion is definitely there at 17mm, but not any more than most other zoom lenses and you'll notice only on certain subject.
This is likely where most people will find the lens a bit disappointing. I find the focus speed to be quite good. Better than the previous version. It doesn't hunt as much. The noise is about the same, not intrusive but definitely noticeable (to the photographer). The focus accuracy is what I would like to see improved the most. I've gotten occasional unsharp photos and what I realized is that if i take 2 or 3 frames of the same shot, one may come out sharper than the other. Many people will mistake this inconsistent focus for the lens being soft, especially wide open. It's not that bad and its only occasional, but it does happen more often than with my Nikons, which almost never mis-focus. You have the option to send it in to Tamron and they can calibrate and possibly improve it (which I'm going to do), but many people may not want to go through that. The reason I'm willing to do that is because I know this lens can be spectacular with just a bit of improvement. You can't get an f2.8 zoom from Nikon for less than $1200.
Sample Variation (a.k.a "bad copy)
There is a chance you could receive a "bad copy". Tamrons and 3rd party lenses in general are infamously known (online forums) to be hit or miss. It's a chance many people may not want to take but they do carry a 6 year warranty and customer service is very good about "calibrating" the lens if you think yours front or back focuses. Again, not everyone will want to deal with that, but I have to tell you that its a small inconvenience when you compare it to the years of great pictures you'll make. If you're really into photography, I think you can live through it.
For those who know the benefit of a constant f2.8 lens, you know how valuable that is. Sure you can get slightly better picture quality from other, more expensive lenses, but the attraction here is constant f2.8 at this price point. It's relatively cheap when compared to Nikon offerings. Also, most of us dont view pictures at 100% magnification, which is the only way to see the lens' shortcomings. You may never see the flaws in a 4x6 or 8x10 print. Most casual to enthusiast level photographers will never know the difference between this and a pro equivalent. I also own the nikon 16-85 VR lens (not a pro lens but exceptionally good) and I would say it favors very well against it. Although, the Nikon is a bit better at any setting, you cant get the shallow depth of field and creamy background blur with the Nikon that you can with the f2.8 Tamron. Also, once you stop down the Tamron to f5.6 or f8, the difference between the two is hardly noticeable. For me it's not all about sharpness, especially when photographing people. You don't want to highlight wrinkles and skin imperfections so I typically soften them up anyhow. Plus, when I want really sharp photos it's also up to the challenge. I've taken some exceptional images with this lens and that is the bottom line. There are trade-offs and you need to know your type of photography to know what lens will suit you best. With Amazon's awesome return policy, you've got nothing to lose. Pixel-peepers and lens-snobs need not apply.