I have notes on more than 200 people at Stars, and they stay where I put them. Pokerstars.exe saves them in a file called "notes.txt". At low limits, there are so many players that it takes quite an amount of note-taking before you end up sitting at the same table as someone again. But keep at it, and you'll soon start seeing more and more of your old pals showing up.
It's very gratifying when you play a hand differently than you normally would have because of a note, and make (or save, of course) money off of it. One of my favorite players is a guy who limps in with 60% of his hands, but raises a small percentage. After about 300 hands with him, it's clear to me that he raises only - but all of - his pocket pairs. I've seen him showdown AK twice, no preflop raise, and I've seen him showdown deuces once - raised preflop. The next note I have on this guy is "when he has a pair, he goes to showdown heads-up" so if he raises preflop, he's going to showdown if I isolate him, no matter what I do. Therefore, I know I can't possibly bluff him out of a pot he's raised PF (and I'm not going to even try), and I know that he'll eagerly call my top pair bets all the way to the river.
I think I may have gotten a bit off topic, but taking notes gets me all excited.