I haven't yet had a chance to formally test the
Tamron 18-270/3.5-6.3 Di-II VC yet, but the reviews I have studied all seem quite positive, given the inevitable limitations of a zoom with this wide a range.
I have tested the somewhat similar Tamron 18-250/3.5-6.3 and you can read my review of that lens here -
http://www.bobatkins.com/photography/reviews/tamron_18_250.html. Overall I thought it was a pretty good lens for a wide range zoom. The 18-270 adds a little to the zoom range, but more importantly adds image stabilization through the Tamron VC system. I recently tested the Tamron 17-50/2.8 Di-II VC (
see review) and found the VC system on that lens to be very effective, giving around 4 stops of stabilization.
I did make some brief comments on the Tamron 18-270 VC when it was announced (see
http://www.bobatkins.com/photography/reviews/tamron_18-270.html) and it still looks to me like a pretty good choice if you want one lens to do everything. Note that there is currently a $50 mail in rebate from Tamron on the 18-270 which makes it an even better deal.