My wife and I have GT multi-speed road bikes: extra large 31" wheels, skinny tires. Really, really fast on paved trails with very little effort required, but very hard and sometimes impossible to use on dirt trails.
We both wish we'd bought mountain bikes with smaller wheels and fatter tires so we could take advantage of more trail types. We're willing to go a bit slower on paved trails.
My buddy is a heavy duty mountain biker, and he's found that he only uses very few of the available gears on his Gary Fisher bike, so he's just bought a single speed mountain bike. Much lighter, simpler, and cheaper than his multi-speed.
Not sure if I'd go with a single speed, but it works for him. Something to think about, anyway.