At my gym we using Schwinn Spinning (branded) bikes. They are essentailly a fixed gear bike with a heavy flywheel, and resistance pads. The other gym (my club's sister club) has Power Pacing (branded) bikes. These are essentially the same thing, but I like them a bit more. They don't seem to break down as often. But my gym keeps them pretty well maintained, so they are usually all up and running.

They are much better than a lifecycle bike or something similar. I think they do a fairly good job at recreating real bikes when the resistance is on.