Dylan Berrywrote a review of Shimano ZEE RD-M640 Shadow Plus Rear Derailleur on November 3, 2014
Familiarity: I've put it through the wringer
I've got this derailleur mated with ZEE cranks and 1x10 speed XT gearing. With a Narrow Wide ring upfront no chain guide and the clutch engaged I have had no issues with the chain derailing and the shifting is smooth as silk. Before installing the NW ring I did have a few issues with the bike spitting the chain, but once I upgraded there were zero issues. So far so good with the ZEE!