i got my 17-40 for cheap so why not? The Tamron's great as well but for a little bit more money in my case, I'd be stupid not to get the L - better build, USM, higher resale value, etc.
It's only an f4 so to some people it may be too slow, but for me it's perfect as a daily walkaround lens... too short? walk closer 
I acutally borrowed a Tamron for a few days to compare to my 17-40, image quality is identical, but some might say the L has better color and contrast... well I didn't see any major difference between the two so perhaps I got a good copy of the Tamron. I also found that 40mm on the Canon isn't that far off from 50mm on the Tamron, you just have to move your body forward a few inches to get the same crop.... so to me that's not an issue. My copy of the Tamron had some trouble doing accurate AF, so in this case the 17-40 wins by quite a bit.
When it comes downt to it, if you can find a sharp copy of the Tamron at $200 bucks less than the Canon, and that you don't need USM, better build, or worry about resale value, then the Tamron is a good lens for sure. But if you appreciate what the Canon has to offer and can find a good deal on an used 17-40, then I say go for the L, you'll love it.