There should be a Moores Law for cleaning robot since they are upgrading too fast that many people don't want to buy it too early.
The 650 is the robovacuum Sweethome recommends and has recommended for years (http://thesweethome.com/reviews/best-robot-vacuum). The newest Roombas since then have been mildly incremental, and the competitors have struggled to offer something that works better. One of my close friends bought the exact same model last week for 370 dollars.
That's not to say there isn't room for a lot of improvement, but I think it's been a super slow moving space.
I also have a Roomba 555 bought for $200 in 2012. I spent about $100 to upgrade the older one to the newer sweeper/dustbin and to buy a new battery.
Love them both! Could not imagine life without them. To press a button, leave, and come back to a parked Roomba with a full dustbin is glorious!
The amazing thing is that they actually do a better job then using a broom and dustpan.
It's running right now, and with two dogs and a cat my floor is rarely ever this clean.
We're waiting for the Apple II or iPhone 1 moment then things will get interesting.