I was lucky to be able to upgrade my camera kit and spent a lot of time shopping and debating.  I originally was going to get the 24-70mm f/2.8 lens, but someone in one of the forums suggest I look at this one.  Though I lost a stop of light, I was very much sold on the IS idea (hands shake) and the cost difference between the two allowed me to invest in the Canon 430 EX II flash, which was something else I really needed to invest in, so I decided the trade-off was worth it for my purposes.I've been using the lens now for about a week, experimenting with the IS, the focus, depth of field, etc.  I wanted to be pretty sure I knew what it could do before I wrote a review.Focus and focal range:  my camera focuses very quickly with this lens, and the full-time manual focus is great for those moments when the camera "guesses wrong" and focuses on, say, the post beside the bird instead of the bird itself.  I was pleased that 24-105 is workable as an indoor lens on a crop sensor camera.  I didn't run into any "have to back up to get that picture" moments so far.  It definitely isn't an ultra-wide lens on a crop sensor body, but can be used comfortably every day.  The zoom works well for the across the room zooms when you're trying to just get the subject's face in the camera, making this a great choice as a walk-around lens for most people photography.Sharpness:  I bought this at the same time as the 70-200mm f/2.8L IS USM lens, so I have a much sharper lens (the 70-200mm f/2.8L IS II is a definite "wow" lens) to compare it to, so I will say this lens is acceptably, moderately sharp without a lot of post-processing.  Not as sharp as the 70-200mm f/2.8L lens (it also costs half as much), but sharper than the kit lens and the 55-250mm lens that I had with my T1i, requiring less post-processing to get a sharp image.  The colors are rich and the contrast is excellent, creating a pleasing image.Build quality and feel: if you are someone coming from the consumer lens category you will immediately notice the difference.  From the weight of the lens to the "feel" of the focus and zoom rings and the more quiet, assured zoom you realize there are reasons why L glass costs more beyond just the research and engineering required to make a higher quality image.  The IS also runs more quietly than the IS in the consumer-level lenses.  The lens is built dust and weather resistant, meaning that it can stand up to a little weather here and there, etc.  Additionally it comes with a pouch and lens hood included.  Unlike what I can research about the consumer hoods, the lens hood on L-glass comes with a flocked interior, which absorbs light better than the plain matte-black plastic.I am very pleased with my choice, and would definitely probably do the same thing again without hesitation.  Now I am debating between the 16-35mm f/2.8L II and the 17-40mm f/4L.  Decisions, decisions... 

Question: What lenses do you prefer?
Answer: The lens is built dust and weather resistant