I have one my 550 and I absolutely love it! It helps me catch any potential double charges by feeding a second case into station one if I happen to pull the handle without rotating the shell plate. I load 9mm, 38 super, 38 spl, 357 mag, 45 acp, 44 spl, 44 mag and 45 colt with mine.
The only real issue I ever had with it was that the yellow case feed adapter for 44spl,mag and 45 colt would not feed cases into the slide at all no matter what I tried, so out of frustration, I tried the orange adapter for 338/357 and it has worked perfectly!
I myself prefer the 550 over the 650. The 650 is a great machine as well, it just prefer the manual indexing.