I bought a single speed mountain bike last summer. I live in SE Arizona and there are a lot of great mountain trails, double track dirt roads and plenty of hills for riding. I've decided that the single-speed is a great way to force yourself to do intervals. I have a eight mile course that is a killer and I spent a couple of months walking parts of it. For the last two months I've been able to ride it from start to finish without walking my bike. I've been working on cutting my times.
I tried it for awhile wearing my heartrate monitor and decided that it was distracting me when my pulse would get high. I took it off and focused on my breathing, pace, standing and seated climbing and I am doing great. I hope that it translates into greater fitness for road cycling this spring. However, I may just keep after this as I'm getting quite obsessed with "cleaning" certain sections of road and trail.