I have the 552, and am very happy with it. I find it very versatile and well suited to the kind of work I do (bag work!), having the recorder is a massive bonus when doing 5D stuff or working with a hop, or even just for general backing up/covering your ass!
I just wish someone would release a nice (small) master clock type thing, as the only thing I miss in my mixer is a time code generator.
Also, if you buy a 744 later on, you can have 6 iso tracks with time code.