Summary: I was using a kit zoom and I was out in on a windy day, when I got home I noticed dust in the View finder. I took the lens off; I saw dust throughout my camera even on the sensor. I had to buy a new focus screen; the old one couldn’t be cleaned. I was lucky to have the equipment to clean the sensor, or I would have to send the camera out for repair. This lens is light and sealed to dust and rain drops. Very easy to use and at (2.8) it’s awesome.
Conclusion: Since I traded the 24-105L for this lens, it's by far my most used lens. Sharp, fast, flexible, 24mm is great on FF. It's big and heavy, but that doesn't bother me at all (I shoot with a 1DM3 + 70-200 f2.8L IS for 16 hour days). The lens hood is genius, providing excellent coverage whether you're at 24mm or 70mm. I mention build quality as a negative, here's why.
Conclusion: I originally had the Sigma EX 24-70mm f/2.8 lens and found the image quality great for the price paid, however it did struggle with autofocus. I paid 3x as much for the Canon 24-70mm f/2.8L, and yes the autofocus was much faster, and while the image quality wasn't that much better than the Sigmas, the colours were cleaner and contrast seemed slightly better.
Pros: Contrast, flare, accurate AF, sharp at f/2.8, solid build
Conclusion: A real workhorse lens which won't let you down but also won't thrill the pixel-peepers. Would really benefit from IS. Range not really suited to 1.6 crop cameras IMO and performs better on FF/APS-H rather than the latest high pixel density smaller sensors. It's heavy for what it is but well balanced on pro camera bodies/gripped cameras.
Pros: Fast aperture, great colour and contrast, good bokeh for a zoom, built like a tank, useful range on FF and 1.3 crop.
Cons: Sharpness at f2.8 not up to the standards of Canons latest lenses, heavy, no IS. Must be due for a make-over soon.
Conclusion: I've had this lens for around 5 years now. It has been my workhorse lens for events including weddings. I really enjoy shooting with this lens cause it has an extremely high keeper rate. I used to have both the 24-105L and this lens. My idea back then was that I'd use this lens for work and the 24-105L for traveling.
Pros: Sharp, Great color rendition, perfect zoom range for a full frame, and expensive.
Cons: Heavy and bulky (but, the sharpness of this lens is like a prime. So, it's just like carrying around 3-4 primes in one lens.)
Conclusion: I got this lens the same day I picked up my 5D Mark II. Coming from a 40D and 7D that I'd used with the EF-S 17-55mm f/2.8 IS, I felt f/2.8 was pretty important to my style of shooting. Shot it for several months on my 5D Mark II and loved it. It's been great for candids and as a general purpose lens but, I think most people would be better served by the 24-105L.
Conclusion: Have lived with this lens now for 2 and a half years - it has become my always-on-the-camera lens; my default. Unfailingly good images, bright vivid colours and sharp as you like. Don't harp on about "when will canon build this lens with IS?" it doesn't need it and maybe that's why it is so long-lived. It's one of my least glamorous bits of kit - but it delivers time and again. The only good reason not to buy this lens is if you can't afford it. If you can - buy one.
Pros: Build quality, image quality, versatility
Cons: none - in a parallel universe the same lens but lighter would be nice...