This thread went dead for a while, but I have been looking at this, as a satisfied user of previous Tamron super zooms. It is true that you have to adjust expectations for a lens like this, but there are times, primarily when traveling, when hauling several lenses and having to switch them out in busy, crowded situations is truly a hassle at best, and impractical at worst.
However, in the past week I have searched for reviews of this lens, and was pleasantly surprised. It seems that Tamron, while extending the zoom range to a crazy extent, may have produced their best super zoom yet. I am not trying to shill for Tamron, and you all can look for the review yourselves, but based on what I have seen, it outperforms the previous models in terms of distortion (easy fix), and is no worse otherwise. Reviews point to somewhat lower contrast (easy fix) and saturation (easy fix) than you might want, and offer the caveat that you don't want to spend a lot of time in 1:1 pixel peeping if that gets your blood pressure up (good advice for anyone with any lens). However, the quality does seem to be truly surprising.
Now, this is not a prime lens, nor a 4x or lower zoom. It will certainly behave better with good light than in dark places. No surprises there. You know if this lens will be useful for you, given the inherent trade-offs, and if so this lens appears to be the best option that Tamron has produced yet.