I find my rigid singlespeed totally superior to my geared full suspension bike. For one thing, it's much simpler because there's no suspension to break, and no gears, which is the key difference that probably nobody has explained to you since the last time somebody mentioned anything about singlespeed mountain bikes.
Without gears, I get a "duathlon workout," in which I run up a lot of hills, at least until I'm exhausted toward the end of a ride - the parts after the first five minutes or so - at which point I sort of shuffle and complain up the hills while dragging and carrying the bike, which is admittedly quite a bit lighter to drag up hills than the geared full suspension rig, which I normally ride up but I suspect would be quite hard to push.
And there's nothing to break, except for spokes and rims which I break a lot because I'm pretty fat and there's no suspension so something has to give when I plow into rocks.
Speaking of which, crashing is easier on a rigid singlespeed too. Rather than bouncing in the air and doing things like "endos" and "high sides" and "coming off," which are hard to describe and must be accompanied by appropriate body english and hand gestures, there's no bounce on the rigid and I simply "eat shit," usually face first and without catching big air, so it's much easier to describe and it usually happens a lot more suddenly, so there's none of that aching anticipation of a big crash on the way down. It also makes for shorter crash stories, so my singlespeed rigid is much less tiring, at least for my friends who have to listen to my stories about crashing.
In the final analysis, singlespeed rigid mountain bikes are superior, because they just are. If mountain biking was a Marshall amplifier, regular mountain bikes would be an "11" on the volume knob but rigid single speeds would be like a "953 to the 7th power" on the volume knob for an entire Marshall stack with AC DC playing live on it in your basement and you have a free keg of Natty Bo to drink while you're listening to them and Bonn Scott gives you his drumsticks and there's a couple groupies sitting on your lap. It's like this because rigid SS'es are just like regular mountain bikes with gears and suspension, only they totally and completely rage on a whole 'nother level. Plus they don't have all that extra shit that can break or freeze up in cold weather, in case nobody mentioned it to you.
Frankly I don't know what people did before rigid singlespeeds came along. I guess they rode geared full suspension bikes, and before that they rode rigid singlespeeds because they were badassss, like Gary Fisher, who basically invented the mountain bike. God knows why they invented geared full suspension bikes; I guess they just wanted the challenge of trying to stay hard as a Mothafucka inhe face of living the luxury life. Now *that* is a challenge to rival the gnarliest hill.
So if you are thinking about getting a singlespeed mountain bike, ask yourself this simple question to determine whether it's right for you: Do I want to be badasssss just like Gary Fisher?
If you want to be just like Gary Fisher, then you *definitely* need to get a rigid singlespeed mountain bike. For sure.