Summary: This flash has been great so far. It is an obvious improvement from the built in flash on my 20D, and has made indoor photography infinitely easier in my rather dark 1930's house. While it can't control other flashes, and may lack the additional range of the 580, the price and features were right for my needs.
Pros: Consistent Output, Fast Recycling, Versatile
Summary: I have not had the chance to use this flash in all situations where I think I will, but I can say without qualification that it does absolutely everything I expected, and as easily as I had hoped. I have it mated to a Canon Rebel T1i and it works great. Recycle time is very fast and I can shoot at least 5 shots in continuous mode without any delays.
Summary: I bought this to use with my new super-zoom. I still have much to learn in terms of using and maximizing all of the features but I am still able to get shots that I would never have been able to capture with a lesser flash. After trying the base model Speedlite, I'm glad I went with the larger "horsepower" of this model.
Summary: I liked this flash. Uses disposable batteries. Charge recycles incredible quickly. As powerful as you might want. No need to get a stronger flash. Fits the hot-shoe of my new Canon T2i nicely. Swivels. It is a bit plasticy, but light. Good photos; wouldn't go higher for portraits or parties. A bit pricey nonetheless, but you probably wont replace it anytime soon.
Pros: Consistent Output, Easy To Use, Fast Recycling, Lightweight, Powerful Output, Versatile