Shimano Zee M640 Rear Derailleur
The Shimano 10 speed Zee Rear Derailleur features Shadow Plus technology that significantly reduces chain noise and offers a more stable drivetrain.
it has been a bit difficult to tune, but i just realized my hanger was bent so that will probably change the tuning. has helped with chain drop. runs very quiet. all in all, shifts well under pressure. the short cage is great for 1x10.
I ran this unit for about 4 months. It had problems shifting in the middle of the cassette, both up and down. It would also flex a bit and pop off the gear if I put too much pressure into it. Needless to say, I often sat down and shifted in fear of slamming to the ground from a chain popping off a gear. Do yourself a favor and get a Saint Derailleur - there is a night and day difference in their performance. If you don't need a short cage, the XT and XTR work great.
I have been using this derailleur for a couple months. Before I used a clutch derailleur, I did not believe all the hype of how good the clutch derailleurs worked. Well, they are right, it makes the drivetrain much quieter and smoother. The clutch on/off lever makes taking the rear wheel off much easier as well. Overall a great product this far.