I recently had the opportunity to test for sample variation from 10 samples as I was very interested in this lens.
Sadly I found that there was very wide variation between units. The problems varied from being very soft all over at all focal lengths especially at the corners, bad centering on many and some were very sharp in the centre but soft at the edges and I found that out of the 10 samples there were only 3 that were pretty good at all focal lengths and across the frame.
Even so, all had a fair amount of CA, very critical sharpness/aperture tolerance (fell away outside of the f8-f11 band) and some distortion at all focal lengths, it also had a tendency to over expose by around 1 stop and had considerably lower contrast and colour fidelity than the Pentax/Samsung D-Xenon 12-24.
So unless you desperately want 10mm or a close focussing distance, I would strongly suggest the latter.
I ended up buying a D-Xenon 12-24 which was considerably less expensive than the DA 12-24 here in the UK, but now very scarce as supplies have all but dried up. It's performance is considerably better than the Tamron or the Sigma 10-20 (I've had two of those). Sadly the Pentax is a horrific price over here.
Basically, if you don't have the opportunity to select a Tamron from a reasonable number of units, I would not recommend buying as the chance of getting a well made one seems fairly low. This is probably why there is a generally poor opinion of the lens from both reviewers and some users apart from a few isolated cases.