On the "negative" side, SS raises the threshold of effort (compared to a geared bike) below which you end up walking. So if you're on a very long ride, or on hills that you're not strong enough for yet, it can be a bummer to end up walking.
When I had knee surgery, I rode a geared bike for a couple of months before going back to singlespeed because I didn't want to walk every little climb.
But for someone who can already ride geared MTB, you'll not lose that much switching to SS and pretty soon you'll develop in the places you need to (beard, general manliness), to be able to SS up nearly everything.
On the positive side: no chain slap, no expensive dangly things close to pointy rocks (ask Phil Simcock about that on the Highland Trail :wink:), you brain can focus on the trail not selecting the right gear, and so on, and so on....