I'm never sure where to rate things on here. The lens is pretty good but it's not the Canon L lens. I like it's performance. It has great image quality and works well on my Canon 20D and Rebel GII.
The reasons why it's not the same thing as the Canon L version is the inability to manual focus during autofocus mode. You may be able to if you set the focus ring to "M" but it will still turn while it autofocuses.
If you want to manual focus, you have to turn the switch and flip the focus ring to "M."
The autofocus is a bit noisy as well. While I don't believe it will be too disturbing, it seems a little louder than most of my lenses like my kit lets, 50mm 1.8, and 80-200 4.5-5.6. It could be because it has more to move and it needs to move it faster to keep up with the quality of the glass.
It's also a little hard to turn the focal legth ring. I have to hold the camera a little harder than normal to change the focal length. This might not be good in quick situations.
The quality probably isn't quite Canon L quality. I guess since it's made for more than Canon, it's probably not quite Nikkor pro level quality either.
I got the lens for just over $400 new at B&H but the new Canon version is around $1100. I don't improvements in the areas listed justify the price but this is still a great lens and I'm glad I bought it. The wider angle of view is excelent as well.