Well, single-speed mountain biking is kind of a subculture within mountain biking, and devotees love it for its simplicityone gear, no derailleurs, fewer cables, no shifters on the bars, and no reason to worry about what gear you're in. Of course, that simplicity comes at a price: having no way to adjust your gearing when you're going uphill or downhill.
Now, looking beyond the cultural aspect of the single-speed, I think they're a great training tool for all riders. They provide a change of pace for seasoned riders, which helps keep cycling fresh and exciting. They're also a great power workout - even though it's relatively unstructured. The intensity is determined by the terrain, and you learn very important lessons about the importance of momentum and planning ahead.
When it comes to building up a single speed mountain bike, choosing the right gearing can make the difference between a fun and effective training ride and a miserable sufferfest. This is where the guys at your local shop may have done you a disservice. A 2:1 gear ratio (like a 32x16) is what most new single-speeds come with. That's a good gear ratio for someone who is riding flat to rolling terrain, but it can be too much gear for someone riding in the mountains or up extended steep climbs. Take Colorado Springs, for instance. We have a park in town that is great for a 2:1 because there are no huge elevation changes. The longest hills are two to three minutes and the 2:1 then allows you to ride at a reasonable speed on the flatter sections of trail.
On the other side of town, we have the mountain trail system that includes climbs that may take up to 30 minutes at a time. For these rides, a 2:1 can be too much gearthe legs fatigue too quickly, riding technique goes to hell, and people get frustrated and hurl their bikes into the woods. Now, stronger riders can certainly ride a 2:1 on those extended climbs, but for those people who struggle with the gear, I recommend going to a 1.8:1 setup (32x18) so you can keep your cadence just a tiny bit higher and make those long climbs tolerable.
My view is that with a 1.8:1 ratio, you can ride long enough - with good form - to have fun and gain the strength and power necessary to graduate to a 2:1.
So, don't give up on that single speed quite yet. Make some adjustments and give it some time. I think you'll come around to seeing it as a nice alternative to riding your geared bike all the time.