I've used them at both extremely low attendance times and in the middle of summer when attendance is quite high. After a few steps of the plan, I abandoned them in low attendance times. You simply won't need them.
When I used them in the summer, I found them extremely helpful. This was back in 94, when there were only three parks, and we were able to finish everything we wanted to do in each park in only one day each. The key is not only following the plan, but understanding the logic behind them so that you can modify the plan for your likes and if any "snags" come up during the day. For that the "cheat sheets" at touringplans.com (the site for the UG) are helpful.
Not to start a "battle of the guides" here, but tour guide mike is probably helpful if you don't understand the logic of the plans from just reading the UG. The logic is the same, but just presented in a different way.
