I do own a 100-400 and find that it is on my camera more than any other lens. Its crisp and clear---especially if tripod mounted--- and yields great landscape/sunset/sunrise colors. Like Brian says, it seems like you never have enough reach when shooting animals. It is heavy and has the slide zoom which takes getting used too. But its well worth it. Like you, I like the idea of a faster lens, so for me the 70-200L 2.8 IS is next on my list.