On the one hand, trainers and spin classes aren't really riding, so you can't expect to start spring as strong as you left off in the fall.

However, they are lots better than nothing. It also gives you the opportunity to work on technique -- there's more to pedaling than just turning the cranks, and stuff like spin classes will help (as well as retain some of your aerobic capabilities). So yes, I think they can definitely be an effective alternative.

One of the down sides is that riding a trainer can be awfully boring.