It’s an interesting question because of spinning’s popularity and the fact that many serious riders enter spinning classes frequently in the winter season.
I believe most people can benefit from spinning classes. Spinning bikes are different to ride than regular mountain- or road bikes, but the physiological adaptations are so close to adaptations from road cycling. Thus it’s almost certain you will improve your cycling performance when you enter a spinning class.
It is impossible to compare 1hr spinning with 1hr Mtb / road cycling. There are many external factors that will influence on your perceived exhaustion (temperature, humidity, music, instructor etc.) Thus, you might very likely subjectively feel you’ve worked harder than you objectively have (a power meter might tell you a different story…)
Also remember that most spinning lessons are made with fitness people in mind. They want to entertain and train people who enter the fitness room once or twice every week, not cyclists who train 8, 12 or even more hours per week. I don’t think it’s a bad decision, but it’s worth to remember when you are road cyclist with a much larger training amount and a different goal with your training.
Spinning lessons have a normal length of around 50 minutes. For trained cyclists this is a very short amount of time, when this time includes both warm up, intervals and cool down. Remember that most spinning lessons are made for people who are not very familiar with cycling training. Therefore, I suggest you do a warm up before you enter the spinning room in order to get more training time.
Spinning rooms have a very high temperature and high humidity. Therefore, it is quite normal to sweat much more than you are used to. Some people believe that they sweat more at spinning lessons because they work harder. That is not true. They sweat more because of the climate, not because they work harder than normally. I will recommend you to drink water with electrolytes and carbohydrates to maintain a high level of performance during the whole session.
Spinning instructors plan their lesson to be interesting and challenging for a wide range of riders. Most of them train spinning only 2 to 3 times week, so they can use all their effort in this short period. If you do not like the program of the instructor, consider to use your own program. It is possible to ride one of these programs without telling the instructor. Just remember to stand up and sit down when the instructor tells you to (and ignore his commands about pacing strategy).
You can’t do intervals every single day. Some days should be easy days. If you go for a ride in the spinning class on one of these days, please remember your goal with the training.