I've climbed on the M with good results. Like any friction device, there are lots of variables that have to be in order for you to optimize the use of the M .
The M is rope-dependant. Be sure to use the right rope with it.
Your body weight might be to light or too heavy to get optimal performance.
One limitation is that, since its a closed tool, you can't put it on mid-line, you have to thread the rope. I never found that to be an issue. Since most climbers who used advanced friction hitches don't untie this is a moot point.
Tom