I have been taking spinning classes and using a trainer for about 2 years. I think if you cannot ride during the winter months, both activities can help maintain fitness and improve. The key is not going nuts riding stationary. In comparison to outdoors, neither can replace the road, but both provide a way to get a very structured workout.
The spinning classes can give you a good workout, but they are usually 45 minutes, not long enough to maintain the level of endurance I want. So, I usually get to the class about 30-34 minutes early to warm up and get a bit longer workout in. The quality of the classes are very dependent on the instructor. Not surprisingly, those instructors with a cycling background are the best IMO. But, I find the spinning bikes do not have a very good road feel. The "brake" type resistance takes some getting used to. That said, I have found the classes (and extra time of the bike) to be a good way to get rides in over the winter and they provide an alternative to spending every night on the trainer.
IMHO trainers provide a better road feel. I personally favor progressive resistance fluid trainers over mag trainers. I would recommed getting some spinervals DVDs or an Ipod to help get through the workouts. I have also found the use of a power meter provides extra motivation.
J