I have one and I love it.
Photoplus magazine gave it a glowing review which gained my attention.
Stuart from digital depot brought along various lenses including this one for members to try at a photowalk.
He couldn't bring the Canon version as he was out of stock so he brought the n.n.n.ik (I just can't bring myself to say it

) version.
I had a play with it on another body using one of my cards.
When I got home I was very impressed with the results.
I didn't want to resize these but have a look at these shots I took using the 18-270mm
http://farm3.static.flickr.com/2751/4143913192_d835075d8a_b.jpg
http://farm3.static.flickr.com/2456/3932155836_0b58dd6111_o.jpg
I know a lot of people shy away from lenses with such a long range but in all honesty, I find it is just as sharp as the Canon 55-250mm IS and the Canon 17-85mm IS I previously owned.
In some case I believe it is even sharper.
I think Tamron have done a good job on the design and engineering on this lens and because of this, I've also recently purchased their new 17-50mm f/2.8 VR and again, I am really happy with the results.
I have absolutely no connection with digital depot but if you do decide on this lens, Stuart will probably be able to match the best UK price you can find
If it's any help, if you are anywhere near north London, you are welcome to take some test shots with mine.