Here's an interesting observation: I shot an event a weekend ago and used over the course of several days: the 135L (and, at times, 135L + 1.4x II), 100L IS, Tamron 24-70VC, and the 70-300L. I didn't even plan on using it, as I consider it too slow for indoor work. But I was surprised to find that my sharpest images from the event were actually taken with the 70-300L.
This was taken from roughly 60 feet away, 300mm, wide open (f/5.6). For this type of work, I find the result pretty stunning. I've included a 100% crop (the first square crop is already reduced slightly to get the square crop for a portrait orientation shot).