It's funny how many people will insist the 650 is a better choice than the 550 (mostly because that's what they bought, combined with egos and whatnot), but the same guys will complain about how hard it is to get the 650 running right.
It's true, the 650 is more tricky to get set up right, especially if you're using a bullet feeder. I had to make a number of "improvements" to my 650 that the 550 never needed. It's finally running right now, after more than a few $$ thrown at it in upgrades.
The 550, on the other hand, is like the little engine that could. It doesn't load ammo as fast as the 650, but it's operation is much simpler and it just chugs right along. The primer system isn't perfect, but it's worlds better than the 650 rotary system, and there's no issues with powder spilling or bullets falling out.