I'm getting ready to buy MST guardian engine flush system, who uses this system and does it flush the out drives? any feedback would be great.

If your boat is sitting in salt water - the drive is going to be full of salt water. I checked out the pics on their site and I do not see how it would flush the drive at all unless the motor was running and exhaust thru hub.
I've Never used MST but I would run Salt Away through it and not just water.