I've loaded many thousands of rounds on a friends 550 and several thousnad on a 650. They are very good machines, however, when I tried the Hornady Loack=N=Load progressive, I liked it better.
I am one of the few who has actually loaded on all three for more than just a casual pull. The Hornady offered more features, had more inovations, had quicker caliber changeover time for shell plate, dies, and powder (around 3 minutes for the Hornady and about 10 for the dillon) and was a lot less money for press and accessories.
The cost of the dillons with three caliber conversoins is at least a third more money than the Hornady with three conversions.
When Hornady gets their auto case feeder on the market, I believe the Lock-N-Load Progressive will surpass the the 650 in sales.