Well, the new Sigma 50-150 f/2.8 APO DC HSM EX (phew!) arrived yesterday so here are my first impressions. I've included a few test shot 100% crops but, before anyone asks, I don't have time right now to do any more - sorry 
The tests are completely unscientific, but enough to satisfy me that the lens is OK and of very good quality.
Initial impressions are good - the lens feels extremely solid and well put together. No, the barrel is not metal, but that's not a problem in my eyes. It feels a little more solid than the Sigma 18-50/2.8 EX, if that helps. It's smaller than a Canon 70-200/4 L but weighs the same. The (internal) zoom and focus actions feel good - rather heavier than on the Canon but not to a bad degree.
Focussing seems quick, and all in all the lens just 'feels' good on my 350D. So it's a thumbs up for now 
The lens comes with the usual good quality Sigma case and hood. Here are some comparison pics against the Canon 70-200 f/4 L, and a pic of the lens on my 350D:
http://www.gcogger.dsl.pipex.com/stuff/lens1.jpg
http://www.gcogger.dsl.pipex.com/stuff/lens2.jpg
http://www.gcogger.dsl.pipex.com/stuff/camera.jpg
Now on to some test crops. As I said, I don't have time at the moment to go out and take proper 'real world' shots, so these are just 100% crops from test images taken from my back garden! I've compared the pics in each case to the Canon 70-200 f/4 L, at f/2.8 and f/4 on the Sigma and f/4 on the Canon. I've not posted f/5.6 shots since the ones I took were more variable (the lenses are so sharp that camera shake at 1/1000 was the dominating factor). It's not a scientific test - these were all handheld (although at 1/2000 or better) and I didn't get the focal lengths identical for each comparison. They are crops from max quality JPEGS with sharpness/contrast/saturation at 0.
Sigma at f/2.8, 150mm, centre crop:
 | HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Sigma at f/4, 150mm, centre crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Canon at f/4, 140mm, centre crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Sigma at f/2.8, 150mm, corner crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Sigma at f/4, 150mm, corner crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Canon at f/4, 140mm, corner crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Sigma at f/2.8, 67mm, centre crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Sigma at f/4, 67mm, centre crop:
| HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO |
Canon at f/4, 70mm, centre crop:
IMAGE: http://www.gcogger.dsl.pipex.com/stuff/can_4_70_centre.jpg
I'm not sure the corner crops help much, as it's so dependent on exactly where the camera focussed.
My personal conclusions are:
It's a damn good lens and, assuming I find no problems, I'm keeping it
The focal range suits me better than the canon, and the f/2.8 is handy for portraits. Does anyone want to buy a Canon 70-200 f/4 L in the UK? 
If we're pixel peeping, I'd say the Sigma at f/2.8 is softer than the Canon at f/4 (as you'd expect). With both at f/4 I'd say the Sigma was a touch sharper in the centre and about the same in the corners at 150mm. At 70mm it's the same story but I think the Canon is better in the corners with both at f/4 - my test shots were not good enough to prove this, which is why I haven't posted them. At f/5.6 I think the lenses are pretty much the same but, again, I don't have reliable enough shots to prove this.
If I find any problems with it, I'll report back on this thread.
