This is an awesome lens and is a real must-have for birdwatching and other nature photography. The zoom range is extremely flexible and the f-stop reasonably fast, but keep in mind that this is a very, very specialized lens. First of all, yes, it is gigantic! This is not something you'll be carrying around day-to-day. It can be very challenging to get sharp pictures at focal lengths beyond 400mm or so. You MUST MUST MUST use a sturdy, high-end tripod and gimbal head. This will also preclude the "downward fall" described in another review. The lens alone is very heavy, and once you toss in a sturdy tripod you're talking 25+ pounds of equipment at a minimum; this is really going to limit your mobility and affect the way you shoot. It takes a lot of forethought to use this lens, but I prefer to think of that as an advantage.
When used properly, this lens has fantastic image quality -- very sharp, excellent contrast. When I say sharp, I am talking L-series sharp, even wide open. The HSM focus speed is fast, perhaps slightly slower than Canon's USM but not appreciably so. The only thing it's really missing is image stabilization (or optical stabilization, as Sigma calls it), which would really help out on those long shots. This is a minor limitation, though, to a great lens. If you have the discipline to learn how to use this lens properly (and if you can afford it!), you'll reap some great rewards.