I bought one last year, it's useful for extra reach, autofocus not fantastic, but workable (I have 5D IIs). Image quality is great, dynamic range seems very good, highlights are preserved and files are extremely workable, viewfinder is an ok size for composing, love the weight - my primary reason for purchase due to my neck being dodgy, I am currently using without a strap, but need some sort of wrist strap for it to keep things off my neck.
I was considering investing (what a curious word) in fuji or sony gear, but for $430 (I'm Aussie) I saved a heap and can use all my existing lenses on it. I used it yesterday for a sunrise seaside shoot and with my 70-200 2.8II the focus missed a fair few times shooting various subjects, but still got lots of ripper shots. I suggest keeping it off f2.8-3.2 for critical work, I've noticed a significant reduction in sharpness compared to my 5ds.
I have achieved shots that I couldn't get with my full frame camera, particularly of wildlife, so that I'm glad I have a cropped sensor again. I also use it for street photography with my sigma 35 1.4 which is a great combo.