I have a 400D and just replaced my kit lens with the Sigma 17-70 on Tuesday.
sigma 18-50 f2.8?
http://farm4.static.flickr.com/3026/2781537486_5fdf9b7651.jpg
100% crop
http://farm4.static.flickr.com/3027/2781537670_da35166f39.jpg
straight off the 20D @ f3.5 with no processing.
I replaced the kit lens on my old 350d with the Sigma 17-70, now it's my standard lens and is always on the front of my 40D.
It's amazingly sharp and fits perfectly in my arsenal with my 10-22mm (for wide angle landscapes) my 100mm macro and my 70-300mm IS for wildlife (and stuff thats, you know... far away!)
2 shots taken with it for reference:
Wide @ 17mm
Full zoom @70mm
did you mean to quote my post??
Looks like it will be either the sigma or tamron then, but which one.......
I think it will have to be the sigma 17-70 the length is good for me, not being f2.8 across it's full range isn't great but ofsetting that against it being nearly £100 cheaper than the f2.8 variants has me leaning in it's favour. It also seems to get good reviews from you guys.
Why not have a look at the second hand value. It seems to me there isn't a great difference in price between the three (sigma 18-50, 17-70; tamron 17-50)