I ride a recumbent at the gym during the winter, using pretty high resistance. Then yesterday I did a 20 mile actual bike ride - first time in around 2.5 months - through some pretty hilly areas and I rode pretty quickly and efficiently. Felt very strong, in fact.
So my thoughts are that indoor bikes can certainly keep you in shape. The thing is most people who use them at gyms just waste their time because they use little or no resistance, and then top it off with a very low cadence. But if you're willing to sweat a bit I think they're a great workout.