So I own this lens since august and I think it is a good lens at a good price. I haven't tried any other SSM/HSM etc. lenses but I think it is very quiet and AF speed is good but not super fast.
It does have CA in some cases but I have had good experiences removing it in PP.
Best of all, it is very sharp - even at 300 mm
I also think it has very nice smooth bokeh.
So here are two examples:

Test shot for Barry, #1 by Thorstensson, on Flickr
Sony A700 with Tamron SP 70-300mm f/4-5.6 Di USD.
Shot at 70mm, wide open (f/4), developed from RAW in RawTherapee - fixed exposure and saturation etc. but no sharpening applied.
Cropped to about eight megapixels.

Test shot for Barry, #2 by Thorstensson, on Flickr
Sony A700 with Tamron SP 70-300mm f/4-5.6 Di USD.
Shot at 300mm, wide open (f/5.6), developed from RAW in RawTherapee - fixed exposure and saturation etc. but no sharpening applied.
No crop.
You can view the photos full size by following the link to Flickr and then right-click the photo.
BR,
Zeb!