My speed and power output (with a Powertap) are pretty close on the road and on my KK trainer.

I do about 30% of my yearly riding on my trainer. They way to make this useful and to keep my sanity on the trainer is to do interval workouts and not long steady state rides (preferably with some training DVD's). I did a 2 hour interval workout on the trainer today (Spinervals Have Mercy) and enjoyed it. I would go crazy though if I had to spin for 30 min as you mentioned in your opening post.