It sounds like it has the same basic functionality as the Peschges
VP-4E Nav that came with my glider. It worked fine as a vario, and the
distance and glide calculator functions worked just fine as long as I
used the distance and groundspeed/wind outputs from a Colibri to set
the wind and distance inputs correctly. I decided I'd just keep using
it until it broke down and then replace it with a nice new LX or
Cambridge unit.

That was eight years ago and the damn thing just keeps going strong!
Maybe you'll be just as lucky with the M-Nav.