What's the difference between the Deebot 600 / 601 and the Deebot 661?

The Deebot 600 / 601 and the 661 are identical mechanically.  The 661 comes with an additional water tank / dustbin attachment that turns your 661 into a robotic mop.  Just remove the standard dustbin, and replace it with the water tank attachment.  Now your robot will methodically mop your hard surfaces - basically doubling the usefulness of your Deebot.