I'm using MG potting soil in my first grow with 8 plants. It burned the bottom leaves on most (classic brown-spot N burns), and 3 plants went into overnute shock (leaves curled down like a claw). I transplanted two of the shocked ones into a very basic no-nutrition potting soil, and left one in the MG to see who, if any of the 3, would recover. One in the no-nute soil looks like it will make it, the other looks a goner. The one remaining in the MG just started to look hopeful today.
I tested pH on the MG several times -- came out 6.8 to 7.0 consistently.
But, the 5 that worked their way cleanly through the initial burn look great and are bulking up every day. I'm probably going to go Foxfarm Ocean Forest next time around, but I might do a couple with a MG mix cut down 70/30 with perlite and an inert mineral soil just for fun. I'm just a Noob, but I've been gardening everything else all my life, and it looked to me that overall, the MG was just a little hot -- cut down a bit, it would be perfect (and at $5 a bag instead of $25 for Foxfarm, a helluva lot more economical).