I've owned the 24-70mm for almost a year now. This lens has been a favorite of many since it replaced its well-regarded predecessor, the 28-70mm. It shoots images that are very sharp and have excellent contrast and saturation. It's sharp wide open and only gets better when stopping down. The USM (Ultrasonic Motor) focuses very fast, and full-time manual focusing is allowed. I've thought about buying a 50mm f/1.4, but the results from this lens are so good, I'm having a hard time justifying the purchase. I've been nothing but pleased with the pictures I get from the 24-70mm. The constant f/2.8 aperture is great for shooting indoors and produces a very nice bokeh (background blur) when shooting portraits. This lens is much heavier than comparable consumer-grade zooms, but I don't object to the weight. I actually like the heft and feel of this lens on my 20D. The only feature I wish it had is IS (image stabilization).
The one thing preventing an unqualified recommendation is the recent release of the Canon 24-105mm f/4.0L IS. The latter lens costs about the same and has some noteworthy advantages. It is .7' shorter, .2mm narrower and .6 lbs. lighter. It has 3rd generation IS that gives you a 3-stop shutter speed advantage when shooting handheld. I know from my 70-200mm f/2.8L IS that image stabilization is a very welcome feature when shooting handheld at slow shutter speeds. And, obviously, the 24-105mm adds an extra 35mm of focal length on the long end.
The 24-70mm bests the 24-105mm in one way: It's a faster lens. That translates into the following advantages: At f/4.0, the 24-105mm cannot stop subject motion blur as well in low-light situations where the 24-70mm's f/2.8 can give you a shutter speed that is twice as fast. Note that IS does not have any impact at all on subject motion blur, only on camera shake on your end. If bokeh (background blur) is important to you, the 24-70mm will have a slight advantage over the 24-105mm given its wider aperture. A wider aperture also helps a camera focus a little better in low light.
The first run of the 24-105mm had a flare problem (see Canon's Web site for more info), and the early production models have been recalled. But the problem has now been fixed. You'll have to consider your photography priorities when deciding which of these two excellent lenses best suits your needs. You would be well served by either.
Update 2-12-12: It's been over six years since I wrote this review, and I continue to use and enjoy my 24-70mm, which is now paired with a Canon 7D. Anyone considering buying this lens today, however, should know that Canon announced on 2-7-12 the successor to this lens: The 24-70mm f/2.8L II. Contrary to rumors that had been circulating for years, the mark II version does not add image stabilization. Canon's USA website lists the MSRP for the new lens at $2,299.00.