I was tempted by the 17-40 f4 L - but ended up buying the Tamron 17-50 f2.8.
I know the Tamron doesn't have a nice red ring, and its autofocus isn't as fast and not as quiet. But I tried both lenses and I found it really difficult to see what the Canon lens was giving me that that the cheaper and faster Tamron wasn't :\
Was going to treat myself to a new lens at the weekend. Got my eye on the Sigma 10-20mm f3.5.
However.... the washing machine decided to go kaput, so ended up with a new washer instead of the lens.
such is life