I know this sounds like a "duh" observation, but it's one of my Great Planner Truths.
The usefulness of your planner depends more on how much you use it than the actual planner itself. I referred to this phenomenon in my post New Planner Super-Productivity (which is when you are super-productive while using a new planner, but that productivity declines when the novelty wears off and your attention wanes).
The more you use your planner, the more (and more) useful it becomes. Filofax and other ring-binder users experience this when they have their planner, projects, notes, information, contacts and reference information all in their binder. (This is a wonderful type of Planner Nirvana that for some reason is so elusive!) When your binder contains everything you need to plan your life, it is very useful. But only if you use it! You can have all this stuff in your binder, but if it stays closed and stays at home, you'll never use it. Despite its inherent usefulness, it's useless to you if you don't actually use it.
The best planner, no matter what style or format, is a planner you use often, every day. It holds lots of accumulated information. It is your go-to place to look things up whether past reference or general information. Whether you use One Book To Rule Them All (a wonderful phrase from my friend Rori @RoriRants) or more than one planner like I do, you should know where to look to find whatever you need. I keep my goals and planning in my weekly Plannerisms planner, and tons of reference, recorded information and daily details in my day per page planner. When I record information in my planners, I always know where to find it again. When I don't write details in my planners, that information is just lost.
I experienced TMYUI,TMUII recently in a hard way. Until this week, we had spectacular hot (for here) sunny weather. This is Scotland, so I tried to maximize every day because for all we knew the sunny warm weather could end any day and we could have rain for the rest of the summer! So I took my kids out and about every day to maximize our outdoors time while the weather was good.
We had a great time, but it resulted in a planner breakdown for me. We weren't home as much as I'm used to being, and when we did get home at the end of our days out walking in the woods or swimming at the pool, I was tired and just wanted to veg. By the end of the week the house was pretty much out of control. After three weeks, I didn't even know what I was supposed to be doing anymore. I wasn't using my planners like I need to. They stayed closed most of the time (which leads me to another Great Planner Truth, spoken by my sister: "A closed planner is a dead planner.").
This led to my Filofax experimentation I wrote about in my post Planner Paradox. I was going for that Everything In One Book Filofax Nirvana situation but failed (again). I realized I didn't need a new system, I just needed to use the system I have! So when I got back to my system that works so well for me, and actually USED it, everything went back to normal and I felt in control again!
What are your experiences with The More You Use It, The More Useful It Is?