yashiguru

I own mine for a couple of weeks replacing a tamron 17-50 f/2.8.
Impressions till now:
- excellent build quality, smooth focus and zoom ring though I like more the ones on 70-200
- USM - fast and reliable focus
- f/4 is slow for interiors (look for a faster lens)
- light around 500g, easy to carry
- the hood is ugly but that's not a problem
- on 400D at the same aperture ( f/4 ) is sharp as the tamron in the center and much sharper in the corners
- exhibits chromatic aberrations wide open even in the center of the frame stopping down helps eliminating them (are gone by f/5.6)
- comparing both lenses at f/11 I noticed much better contrast and sharpness from canon (lower degradation caused by diffraction)
- the tamron at 17mm is a little wider than the canon

hope this helps