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On a DSLR with 1.3x or 1.6x cropping, such as the 10D or 1D, you will see no difference in the images produced by this lens and the twice-as-expensive EF 16-35 f/2.8L. So if you don't need the extra f-stop, go with this one and save yourself $700.

On 35mm film or the 1Ds, you'll see more vignetting with this lens, but the images are still excellent, and this lens has less flare than the 16-35.

The 16-35 is marginally sharper near the 35mm focal length, and does have much better bokeh, if that's important to you.

If you take lots of wide-angle shots, this lens is a must-have. It's one of the less expensive "L" lenses, making it a relative bargain.

If you get paid for your photos, you should own the f/2.8L instead. For the rest of us, this is a fantastic lens, my second Canon L lens after the 100-400. Construction quality is first rate. Images are sharp. The range of the zoom is excellent. For film. On an APS-C sized sensor (Canon 10D or DRebel) this isn't an exciting range and is only marginally wide angle. Though the Sigma 12-24mm is slower and (on paper) lower resolution---that might be the lens of choice for this range on a digital camera.