If you are going to be riding a bike trainer indoors over the winter to stay in shape then you will quickly find that it is extremely DULL to ride indoors. You don't have anywhere near the sensory stimulation that you have when riding outdoors and a... 

If you are like many cyclists you would rather keep riding through the cold winter nights as opposed to joining a gym. That's good thinking but if you are new to spinning or indoor bike trainers, then you will quickly see that it can be quite boring....