I've been keeping a training log for about 1 1/2 years. Starting a training log was a CRITICAL turning point for me. Before, my workouts weren't as focused. I'd waste time figuring out what weight to use for what exercise, or what exercises to do.
Once I started planning my workouts beforehand, I was a lot more efficient. I was also a lot more motivated. If I ever found myself wavering on whether to go to the gym or not, I'd think about that workout I'd written out. I am a list-person, and I like checking things off. The thought of leaving that whole workout blank guilt-tripped me into the gym.
I also like looking back over past workouts and seeing my progress.