Jason: I made the mistake of using this lens handheld... Shot a night football game, and got some great shots with it, but as the joke goes, "man are my arms tired!". Color and clarity are amazing. AF is extremely fast and very accurate, even on my older 40D. Well worth the cost to rent if you periodically need a big gun.

Alison : I have to send this lens back today and I am already missing it. I rented it for a trip to a wildlife refuge to take photos of migrating snow geese, eagles ect. Lens is super sharp and fast. My images look incredible. Low light conditions are no problem for this lens. Beautiful boka and sharp focus. I used it with the 1X canon teleconvertor and it was steady and sharp handheld. I would say that a tripod is necessary for the 2X teleconvertor and really long shots. Excellent lens for wildlife and bird applications. Loved it!