OK, coming on from the Weight training thread here ...
The first interesting effect to discuss is the one brought up in that thread. Does the overall self-motivation you have used to sign up and plan out your gym sessions equate to having enough self-motivation for each individual gym attendance?
I think not. Some days are different to others and some days you just feel great before going to the gym and some days you really don't feel like going. It's very easy to psychologically fall on the side of doing less, imho.
I also think that each gym attendance makes me feel great afterwards, so they're all worth it, irrelevant of what I thought going in, which ultimately helps me personally to go every time I'm expecting to as I use what I know I'm going to feel afterwards as motivation.