I keep an aeroplane on an airfield where, to get to it, I have to walk past the quite large C152 & C172 fleet of the local (very well run) commercial flying school.
About 50% of the time I walk past, at-least one aeroplane's been parked with the flaps left down.
Equally anecdotally, I've a little experience as a Class Rating Instructor, and a lot more experience checking pilots out on syndicate aeroplanes. In my experience private pilots who don't use either a checklist or a clear mnemonic for their checks invariably miss something important sooner or later. I can't say that I see a great difference between the two from the perspective of completeness - it's the people who don't use a reasonably rigid system that get it wrong. However again, in the air, you really can't afford to be spending a lot of time reading.
I've seen a lot of microlights where the owners have dynotaped the main mnemonics onto the edge of the instrument panel - just the letters, nothing else. Hard to argue with the logic.
G