I use the original straps from the M9 and M7 and I like them a lot. I like the smooth feel of them, which is far better for me than the rubberized backing of most third party straps, and the metal rings and protectors are very well designed. I also like how flexible they are, able to be folded up neatly and stowed with the camera when its in the bag.
I am not so fond of the plastic shoulder pad, though. Although it works, I find that it catches on things like car seats, and I sometimes find myelf untwisting i to make it sit on my neck properly. I am considering the A&A silk strap, as it should be at least as smooth as the Leica strap, and has no shoulder pad.