Just read that you can get it for 35% (that's what.. 250 USD?..) I would definitely go for it then.
Depends upon whether that's 35% off of the new price... or 35% of the new price. One is a 35% discount, the other is a 65% discount.
It also depends upon what the "new" price being referenced... Here in the U.S., the 17-40L is currently offered new on Amazon for $739, which Amazon claims to be a 47% discount off a $1400 list price for the lens. But, Canon's U.S. website currently shows an $839.99 list price, in which case the Amazon price works out to be more like a 12% discount.
Personally, I wouldn't buy the 17-40 for use on a crop camera. Not that it's a bad lens in any way... just that it's designed for full frame and there are others that are a better match for a crop sensor camera.
Instead I'd get one of the others that have been recommended... The EF-S 17-55/2.8 IS would be tops and offers a bigger aperture, but is more expensive. The EF-S 15-85 IS is less money, more compact, a better match for a crop camera, nicely wider and longer, with every bit as good image quality. On a tighter budget the Sigma 17-50/2.8 OS might be a good choice. The least expensive Tamron 17-50/2.8 non-VC rivals the Canon 17-55 for image quality, but lack stabilization (no big deal on these focal lengths) and doesn't have as fast AF.