I also take a lot of notes, and by now it's rare I sit at a table without at least 1 person I have a note on, usually its 2-4 out of 10. That's after about 14k hands at my current stake. So I'd say there are enough refulars to make it worthwhile.

The notes are on valuable or just unexpected things, like someone with 16-20%VPIP limps and calls a 4bb pfr with A9o, or "gets married" to TPTK or overpairs, and as many lines as possible for stuff like a set or a draw, etc. After a couple notes and enough stats I write a summary at the top like mega call station, or smart lag or whatnot like others said already. If you play a lot, notes give you a bigger edge so you can make higher EV moves.