Canon 16-35 2.8 vs Sigma 18-35 1.8

I'm working with a Canon 7D camera at the moment, for a mix of photojournalism and corporate videos. Recently I've been shooting various events and property videos and have been hiring the Canon 16-35 2.8 to do so but I'm now at a stage where I can buy another lens and obviously my first thought was to buy that one. However today I saw Blunty3000's review of the Sigma 18-35 1.8

Has anyone used the Sigma 18-35 1.8, how does it compare?

I know a few people will suggest the Canon 17-55 for a crop sensor camera like the 7D but build quality is important to me. I've had to shoot in all sorts of environments before so I don't want to limit myself with a lesser quality build. I can't find many people talking about the build quality of the Sigma 18-35, is it any good?

My other option is to get a Canon 16-35 mark 1 (instead of the mark 2) which I know has issues around the edge but I hear it doesn't make much difference on a crop sensor. This could save me a fair few hundred (GBP).
I'm aware these lenses don't have IS but when this wide I've not found it a problem yet, what little shake I may get shooting handheld I can fix quite easily in AE and to get the same build quality as a Canon L lens I generally have to go for an f4 lens (with IS) which isn't great for my photography work.

Any thoughts on this would be greatly appreciated

I think somebody here posted some thoughts on the Sigma. The lens is really on the new side so it's a little soon for folks to give feedback. I wish I could.

It's getting more than rave reviews: http://www.lenstip.com/374.1-Lens_review-Sigma_A_18-35_mm_f_1.8_DC_HSM_.html

If I was still on crop - this would certainly be one I'd pick up - over and above the 16-35. I have no doubt it's sharper and obviously it's faster. The 16-35 is well built, but hardly amazing. Optically it's a little better than the 17-40, and one stop faster.

I'm a lover of Sigma lenses - in fact I was just posting a slight disappointment that my new 24-105 isn't nearly as sharp as my Sigma lenses. They have upped their game and their recent lenses are really shaking things up.

The only reason I could think of for a 16-35 would be if you need a fast lens on a full frame, period. Oh, and some folks have to have a red ring! I picked up a 17-40 since I don't need f2.8 for a UWA, if I did I'd be looking at the Tokina 16-28 f2.8 instead.

To me the choice is a no brainer for the 18-35.  Cheaper, sharper and faster at a cost of 2mm on the wide end.  It's not like either lens would eliminate the need for an actual UWA lens at least for most people.