If you use a MTB rear derailer, you MAY want to think about getting a new RD for your parts bench. Shimano has gone to a low-normal for their mtb RDs. That means that when cable tension is released, the derailer moves to the LOWEST gear in the cluster (big cogs). Previously they worked like Road derailers, a hi-normal, release the tension, it moves to the highest gear (smallest cogs).
Now, you may ask "Why is this important to me?"....
Well, I'm waiting......
Ok, I'm glad you asked. This means that the shifting pattern is just opposite. If you're using brifters, sweeping the lever will move you to a HIGHER gear, and hitting the paddle moves you to a LOWER gear. This is going to be just backwards of how you are used to it working, and if you also ride a bike with a road group on it you're going to have to remember 2 different patterns depending on which bike you're on. For me that's going to be recipe for disaster on a hill. If you run barends, it won't be as bad, but the pattern will still be reversed.
The Shimano M750 is the HIGH NORMAL RD. From what I've gathered from the MTB forum the High Normal are pretty much discontinued except for the Deore model. Now they are all Low-Normal (aka "Rapid Rise") I think I'm going to head over to e-bay and grab a few of the older XT RDs while they are still plentiful.
Just my thoughts,
Who realizes that he's been working as a tech trainer WAY too long, based on how he writes
*Edited to correct a techical boo-boo