I used to ride my trainer a lot (I even liked it), but last year I used it zero times. Same goes for this year.

If you're going to use a trainer, I recommend using workout DVD's or at least a structured workout. From looking at numbers from different rides, I can tell you I worked a lot harder when I was using a training DVD. The 5 1/2 hr ride was actually a simulated century on a trainer. Having done more centuries than I can keep track of including many in the mountains, I can say that the one on the trainer was one of the most physically punishing rides I've done of that length -- I could barely stand when it was over. But I can say for an interval set as well. In general, I find riding on a trainer is harder work than riding in the real world.

The nice thing about trainers is that since the environment is controlled. You never have to worry about hypothermia, getting delayed by mechanical, having to fight headwinds or other stuff. As a result, you don't have to hold anything back.