I'm a regular spinner, and have been for a few years. I've found it has really helped with technique - my average cadence is significantly increased for example. But above all, at my gym, they are great fun, especially the ones taught by a cyclists.
As others have said, there's a significant difference between a gym bike and a spin bike, and both are different from a cycle trainer such as Wattbike or Kettler Racer S. (I thinking of getting Kettler for home use as it'll take less space than a turbo set up and is quieter than a Wattbike). The high end machines like the Wattbike can help with pedalling technique - getting left and right leg balanced etc. by training with the power meter.
Being able to mix it up - keep riding through the winter, spin classes, cycle trainer at home - helps with motivation too.