I have the 670 and can't live without it. Best remote I've ever had. Programming by USB is a godsend.
The Harmony 620 is functionally identical to the Harmony 670, but comes in black instead of silver–black. The 670 can utilize up to 15 devices, where the 620 can only utilize 12 devices.
Here is the 670's details.
The Harmony 520 is a mid-range remote that is similar in functionality to the Harmony 670, but with a different button arrangement and a squared-off physical design compared to the hourglass design of the 6xx series. It has a backlit monochrome LCD screen.