Using a Bike Trainer: got any suggestions?

This is a picture of my bike on my new trainer Erin got me for Christmas this year. I have used it a total of 3 workouts now, and I suppose you could say that I am beginning to get into the 'spin' of things. Some things I need to work on...
1. How long should I plan for each workout?
2. Should I base my workout on cadence, or distance traveled?
3. How important is it for me to stay in true riding position/form?
4. Will this actually make me a better rider, or just a guy who sweats a lot while watching TV in the living room

Any insights?