A few months ago, I traded my Tamron 28-75 2.8 (which I had used for about a year) for a Tamron 17-50 VC. Here was my first impression of the lens. After having used the lens for about 3 months, I have a clearer understanding of this lens' strengths and weaknesses for family photographers like me.
1. Versatile and useful focal length. On a camera with a 1.5x crop factor, the range is similar to a 24-70 on a full-frame camera. It's wide enough for scenery and tight spaces, and just barely long enough for portraits. Without question, it is more versatile than the Tamron 28-75 (on a DX body with 1.5x crop factor). Sample shots (with no postprocessing) at 17mm and 50mm:
17mm, f/4.0, 1/250, ISO 800
50mm, f/2.8, 1/80, ISO 200
2. Constant 2.8 aperture. Nice for low-light capability, shallow depth of field (at longer focal lengths), and extending the range of bounced or modified flash.
3. Good resolution and contrast. Regarding resolution, check out a Photozone review of the Canon version of this lens.
4. Decent resistance to flare. Photo below is unretouched, with the strong sun shining into the lens in 120F weather.
1. Noticeable barrel distortion at 17mm. More apparent with some subjects/images than others.
2. Vignetting at f/2.8. When used wide open at any focal length, there is slight vignetting at corners.
3. Cannot be used on a full-frame camera. This lens is only for APS-C sensor size bodies.
1. Vibration compensation (image stabilization). It's effective but not perfect. When it's active, the image visibly becomes more stable as seen through the viewfinder. However, the image doesn't become completely stationary. Instead, it moves very slowly toward a particular direction.
2. Focusing on even moderately fast-moving subjects is not perfect. I'm not sure this is purely a lens issue (as opposed to a problem with technique or the camera), but when I shoot a fast-moving subject, especially on Continuous-servo AF mode, the focus is usually a bit behind the target.
(shot taken on Continuous-servo AF mode)
One of the reasons I hesitated to buy the non-VC version was the supposedly poor quality control (some copies reportedly had front-focusing or back-focusing issues). With this particular lens, I had 2 copies to test, and both focused identically. The first one I got front-focused on my D80, so I got a second copy. The second one front-focused identically on the D80. Soon thereafter I got a D300. Both copies focused correctly at all focal lengths on my D300 without any micro-adjustment. Two copies of this lens is hardly enough to generalize, but for what it's worth, it appears that there were no focusing problems with the two random samples I received, which both focused identically.
Conclusion: for photographers who didn't want to spend the money for the Nikon 17-55 f/2.8G ($1339) or Canon 17-55 f/2.8 IS ($1079), Tamron had a well-regarded alternative, the Tamron 17-50 f/2.8. The problem was that although it was found optically superior to its competitor, the Sigma 18-50 f/2.8, there were reports that some copies didn't focus accurately. From my admittedly limited experience, that sample variation doesn't seem to be an issue with this lens. In my view, that quality control is the more significant difference between the non-VC Tamron 17-50 and this lens. The trade-off is the more pronounced barrel distortion (the non-VC version had barrel distortion as well, but to a lesser degree).
As for the VC feature, to be honest, I don't recall counting on it much. When light conditions are dim, I almost always use a flash, and the subject is frozen by the short flash duration. The VC works though and may have contributed to additional sharpness in my pictures taken at slow speeds.
50mm, f/2.8, 1/20, ISO 800 (flash bounced from nearby wall)
If I had to do it over again, I may have purchased the non-VC version instead. With the D300's micro-adjustment function, I probably could have corrected any focusing issues. If I still had the D80 though, or a camera that didn't have micro-adjustment, I probably would not want to take a chance with the non-VC (notwithstanding Tamron's 5-year warranty) and would stick to this VC version instead which appears to have no focusing issues. Moreover some newer cameras such as the D90 have a built-in distortion correction (the D300S does as well, but not the D300), which might ameliorate the barrel distortion of this lens, though I wouldn't know for sure because I haven't tried using it.