I purchased this lens as a faster alternative to the 100-300 Canon 4.5/5.6 I was using. It was my first L glass. I've now been using this lens regularly for about 1 year.
The focus speed, image sharpness and overall performance exceeded all expectations.
2 Surprise benefits:
1. I discovered I didn't miss the zoom. I have mostly used the lens for shooting motorsport and air shows. Everyone else shows up with a zoom. Most of the time you shoot far at these events. This non-zoom lens is much faster and lighter than it's zoom equivelents.
2. There are lots of show-offs at events like this sporting their bulky 70-200 2.8 white lenses. I had the same reach and same performance without having the "Hey, come steal my lens" advertisment.
After my first event I bought the Canon 1.4x extender. Although I lost a stop - the lens continues to excel!