I agree Lightrules. I have both and the Tammy really stand its own.
Dont get me wrong, the "finish" is obviously clear to see but let us use the lenses they are supposed to.
The 24-105 is a quality walk around lens and just perfect for portraits.
The Tammy 18-270 is ideal for cases where you don't feel like changing lenses or don't have the time for it. Ideal for travelling purposses and random shots.
I used to have the Sigma 18-200 (no OS) whith pretty dissapointing results. It got away pretty quick. The Tammy surprises me every time.
In case you were wondering I use a XT-Tammy combo at work (no time to switch lenses around).
For home I'm happy with ALL Canon gear like a 30d, a 50d, a 10-22, a 24-105, a 100 macro, a 300 and to top it of a 500 reflex lens (but technically there are more mirrors than glass so not really a lens to mention in this collection).
Good luck with your Tammy
Greetings from Ghent,