This seems to be one of the more common questions, but I still haven't found my answer yet, so here it goes: I currently own an xsi and I plan on buying a new "general purpose" lens in the next month or two because the kit lens just isn't cutting it. I already have a Sigma 10-20mm, Canon 55-250mm, and I recently purchased a 50mm 1.8 (super sharp!). Anyway, although the prime is great, it offers no flexibility. Also, super wide and telephoto are both fun, but they are limited as far as general purpose goes. I've read a lot about the 24-105 not being the best lens for a cropped frame camera. Although I understand why, I would be pairing it with a 10-20, so the wide end is not so much an issue anymore. However, in situations where I don't have the time to change lens back and forth, this could become a problem. I just recently considered the 17-40 as an option. This would eliminate some of the wide angle problem if I chose not to carry my 10-20 on me, but it would really limit me as far as reach is concerned. Also, I don't really trust the image quality of my 55-250 enough to completely rely on it for all of my telephoto needs. However, sample images from the 17-40 seem promising and on a cropped frame the 17-40 isn't a bad all-around range. Plus, the price is definitely right compared to the 24-105. I'm sure I could work around either lens. Photography is a hobby for me, and a small mistake in making this decision won't hurt me too much in the long run...I like to believe that my skill as a photographer could make great pictures happen with either of these lens. However, it never hurts to make the right decision the first time! Other things to consider...I usually carry a small bag with a second lens (this would be an argument for the 24-105 because I'd have my 10-20). However, I want one of these lenses to make it possible for me to walk out of the house with only one lens. If nobody can come up with an "all purpose" solution, a recommendation to supplement what I already have would be appreciated as well! Also, I'm pretty set on the "L" series...I see myself upgrading to a full frame in the future.