Been using Mountain Kings for years, both the old design and the new. I just use the bog-basic folding version and the only criticism that I can level at them is that they seem to wear quickly.
Other than that, no issues - a lot of people seem to feel that they puncture easily, or that the sidewalls are prone to tearing but I can honestly say that I've never had these issues (except one of the old ones which blew out but I'd been kicking the arse out of it anyway and it was due replacement).
They're possibly not the grippiest tyre on the market but they do roll quickly, even the 2,4" version. The only time I swap them is in summer when I put a set of Maxxis High Roller Semi-Slicks on if the ground is dry and hard-packed.