I think the M13's effects sound great. They are the same effects that are in the HD500, but it's easier to achieve unity gain to your amp with the M13.
Here's a video someone made up a few years ago comparing the distortion effects in the DM4 to the originals they were modeled after. They're all pretty darn close, if you ask me. The distortions on the M13 and the DM4 are the same.
I have the M9, but the effects are the same. Many people seem to slag the overdrives/distortions on it. I've been A/B'ing to a vanilla re-issue tube screamer and a BYOC overdrive pedal. I'm running it all into a Peavey Classic 20 which for these purposes I'm running fairly clean. I think the M9 Screamer and Tube Drive patches, with some small adjustments, sound very very good. I think I will wind up sticking the BYOC in front of the M9 and leave it at that. I also compared some of the fuzzes to a Big Muff russian I have. There too I'm equally satisfied with the M9.