Whats the virdict

whats the virdict on the tamron 17-50 vc lens? Is the canon 17-55 is really worth the extra bucks? looking to purchase one of these lens in the next three weeks.
I dont want to purchase the tamron only to regret it later.

Also looking for a longer zoom, i already have 70-200 f4 is but find this a bit short for airshows and sporting events even with a 1.4 converter on.

Been looking at the canon 100-400, its resonably priced (compared to the canon 17-55) and seems to be the best zoom to get.

I read some reviews that the Tamron VC isn't as good as the non VC version, thus I would say the Canon 17-55 is worth the extra.
Are you planning on keeping your 70-200? If you are, then you might be better off investing in a long prime like the 300 f4 or 400 f5.6.