I was riding on the street on my way back from a mountain bike ride last weekend and passed a guy on a fixed gear going down hill. He was spinning so fast he couldn't stay on his saddle because he was bouncing around so much. I seriously thought he was going to fall off his bike. It was pretty funny.

Unless you are a very strong rider, you're not going to keep up with any road bike with gears. You might for a while, but you're spending a lot more energy to do the same amount of work, so you won't last long...