Hi,
I have been reloading on a 550 for 3-4 years. If I could go back in time, I'd get the 650 or 750. At some point I may make the change or add a 750. I don't do as much shooting as I'd like, but look towards a time that I can do more. I reload for 45 Colt, 45 ACP, 45/70, Ruger 375, .38 special and at some time in the future several more rifle and pistol calibers.
My two challenges with the 550 are a lack of available options to aid in automation, and the way the primers are installed in the cartridge. 750 solves both those issues and overall makes it a bit safer or more foolproof in my opinion. With that said the 550 is a fine machine that I've been able to safely load north of 6,000 rounds on so far.