This evening I went mountain biking with my dad on the singlespeed (no gears) mountain bike I just built. We went to Addison Oaks, which is mostly technical singletrack through heavily forested rolling hills, with some logs and the occasional rockgarden, much more heavily wooded than most of the trails I ride.
I had complete confidence in the drive for the first time ever. I could get out of the saddle on the climbs and hammer so hard, with no chance of gear slip, and no bobbing from suspension--the bike would just take off and go. The frame is terribly stiff, and my fingers got numb during the ride. Toward the end I got on my dad's geared full suspension bike. The drivetrain seemed so indirect and fussy--I lost speed with every shift. I was high up in the air (the bike has a high bottom bracket to allow suspension travel) and out of phase with the trail. There would be a compression, but the bike would continue going down for a fraction of a second after the bottom of the dip, and when I would brake, the bike would dive and transfer weight to the front wheel. When I got out of the saddle, the suspension would compress on each stroke before I would go forward. But it was a very soft, compliant ride.
I do miss the big-ring downhill rush of having gears, but can certainly get along without the uncertainty of drive during shifts, the noise and drag of the gears, and the cost of all the bits. Singlespeeds are meant to be pretty much silent, but my handlebar was creaking (unnerving) and my rear brake cable is routed along the top of the top tube and goes "ding" to alert me of any trail imperfection..
About that big gear bit- With only one ratio (36 x 16, and the bike can still freewheel, it's not a fixed gear) you can't really pedal on the downhills, and have to pedal every time else, especially right before and during uphills. This means that you are slow on downhills, normal on the flats, and fast uphill. Which means you are always going about the same speed, but with wildly inconsistent effort. On a geared bike, you go all different speeds, but with consistent exertion. Singlespeed is definitely faster for these sorts of trails, since you aren't hunting around for all the gears and are led away from laziness on the short rises.
The bike manufacturers had me tricked for years. Never did I need a bike with all those gears.