Correct me if I'm wrong, but a typical, real MTB does NOT put you in an upright position. Straight handlebars tend to make you lean forward putting more body weight over the front wheel, which works well for controlling a bike over rough terrain. But this is not needed for street purposes and makes it awfully uncomfortable if you ask me. Some less expensive MTB-styled bikes do have more commuter friendly handlebars though, with a rise in them that helps you sit more upright.

I tried commuting on my mountain bike, putting fenders & slick tires on it to improve the ride. But that 26 inch wheel just doesn't cut it for me. I feel like I'm not getting very far for all my effort. (cue the wicked witch theme music from 'wizard of oz'). If I ride my ancient old road bike I can get there & back without feeling like I've killed myself.