I go to spin classes every so often at my club. They have Keiser M3 bikes with totally inaccurate power meters but they do connect to my HR monitor, so its kinda interesting to only ride via RPE.
The ‘feel’ of the crank is like wet oatmeal, and the saddle is like a Huffy bike from 1983. Sometimes the music is good. Usually the scenery is good. 
But no matter what, you can get a good cardio workout, despite a fairly unimpressive power-based workout.