I couldn't agree more with Botto and merlin. Fixed gears are a great alternative, especially in the off season. They are fun and get you out more. If you want a smoother spin, they aren't the best way to do that. Merlinextralight is right in that the momentum pushes you through dead spots. Rollers or one pedal exercises on a stationary trainer is better for that.

I like riding my fixed gear late Fall, the Winter and early Spring when riding takes some extra effort getting dressed. Instead of putting on all the gear, I just put on my helmet with whatever I'm wearing (jeans and a sweatshirt) and go riding through the streets around my house. I play around seeing how far up hills I can go without having to stand and how fast a cadence I can get downhill. 15 miles goes by quickly.