I got this lens for about $150, in Nikon mount. I have used it on both film (F100, F3) and digital crop camera (D7000).
I shoot fast primes, but I wanted a decent "walkaround" zoom that:
1) would cover both wide(ish) and tele end on both full frame and digital crop, wide end being more important
2) is full frame
3) has the aperture ring
4) is cheap.
This pretty much narrowed down my options to this Tamron and Nikon 28-105. What swayed me towards the Tamron were the extra 4mm on the wide end, and reviews that said those 4mm were actually good. The Tamron was also a bit cheaper, but that difference was too small to matter ($50).
For a slow-ish walkaround zoom, which means it's not used as a specialized lens and you don't do pixel-peeping, it represents an excellent option. CA and flare can/will be a problem. For the price you get A LOT.
It focuses close, not sure how close but its MFD must be under 20 inches (0.5m). It works fine at that distance, though it's no macro.
Sharpness: It's good wide open, but it's much, much better when stopped down one or two stops. I usually shoot it at f/8 and f/11 and it works great there. The way I shoot, especially with this lens, sharpness in the corners is rarely of critical importance so I can't say much about that except that I have never looked at an image and said to myself "OMG this lens sucks in the corners". Take that for what it's worth, but I have zero problems with the sharpness of this lens.
Distortion: I don't remember what studio reviews say, but distortion has never been a problem for me. If it's there, and I am sure it is, it's not bad at all and I doubt it's of the complex variety.
Vignetting: I'll admit to rarely noticing this problem unless it's very bad. I've shot many landscape shots on film and never noticed this problem in the skies.
CA: Problem #1 of this lens, I think it should have "CA" included in its long name. You have to be careful shooting with it. I stopped using it on days with strong contrast, because I have lenses that perform better in such conditions. I think CA are worse at the wide end than at the long, but won't put my money on this claim as wide end [24-50] is where I used it the most. Things improve if lens is stopped down, though the problem doesn't go away.
Flare: Well, yeah. Problem #2. Like I said, this lens doesn't appreciate the Sun, so be careful.
AF: Reviews say that AF is slow, which could be true. It's a walkaround lens and I don't shoot BIF/sport with it, so I don't care for its AF speed. It's certainly fast enough for my use, which includes small kids running around in unpredictable ways.
Build quality: The plastic around the barrel doesn't feel cheap at all, but the weight of the lens requires something more sturdy if you want to hope that your lens will survive a 3 foot fall on concrete.