I had overlooked this lens and used a 100-400 IS for a couple of years, and thought it was great as a single-lens solution. But eventually, I decided I needed something sharper and faster focusing, and this was the answer. The fact that it's lighter was originally a secondary consideration, but after using it, I wonder why I wrestled that 100-400 around for so long.
Fanatstic for shooting birds in flight, especially if you remove the tripod mount. The slide-out lens hood is very convenient, especially when used with a polarizer. There's probably no better 400 for handheld shooting. The only caveat is that you don't have IS, so you need to keep the shutter speeds up. And of course, it's only f5.6, but that's why it's so nimble.