Very good lens, I've had two of them before I bought the "L" .
The advantage of the "L" is that it is much easier to use, so if your subject is going to disappear in a few seconds, you can capture it while the USM would still be hunting for focus. If you can be deliberate and take your time, and use a tripod or very fast shutter speed, the USM IQ is actually a tad better.