When I got the M8, I borrowed a 135/2.8. I had had one in the past, but sold it because I used it so seldom.
I liked it so much on the M8 I bought another one.
I have not tried other 135s on the M8 but the convenience of the 135/2.8, plus the VF magnification, is such that I don't really feel the need. The 135/2.8 can take better photographs than I can, so I'm not going to fret about further increments in picture quality.
Cheers,
R.