I have had a 550 for about ten years now and have loaded thousands of rounds on it, mostly pistol, so I guess I will chime in with what my experiences have been.
I was interested in producing good ammunition at a reasonable rate, and once my dies are set up and adjusted I can easily load 400-500 rounds of ammo an hour, so I have more than enough production capacity as far as I am concerned. Auto index sounds like a great feature, I obviously don't have that luxury on my 550, but I really don't mind not having it, it gives me a chance to do things at my pace, that extra step hasn't bothered me in the least. I supposed if I were to try a 650 I would see what I was missing and would want it, but I am content with my 550.