Whoa, I'd be lost without a workout log. I've kept one since 1989. I've used regular calendars and cute little journal books. Either way, it is beneficial just as the girls above have already posted.
I'm currently using a calendar with wide enough boxes to fit in my workouts. In addition I'm using a spiral notebook that I fit 4 days to a page. I write the date, what I ate that day, my workout and notes about my progess. My diet progress under the food heading and my workout progress under the workout heading.
I put in my weight and measurments, my goals, what foods I can have.
Since I started out the year with the new Series, I also inserted pictures of the new workouts. On the facing page I wrote the name of the workout, for example Cardio & Weights. I put in the weights that Cathe uses, the weight I started out with and where my weaknesses are in the class. I also wrote down the Titles of the music. It helped in the beginning to see where I was on the tape.
I also put in what the crew did to prep for the new series, as this was a big motivation for me.
Connie, I love your worksheets. That is a wonderful idea of being able to see what body part you worked during the day and see at a glance what you worked all week. Thanks for posting your log.
This is probably way more than you asked for.

Bottom line, whatever form a log takes, it is very worthwhile to have one.
Wanda