When I first started playing online, I did try to keep a lot of notes on people and they did help sometimes, but I also noticed that some of the people had changed their playing style so, when I was ready for a fish, I would sometimes discover that they had grow up into a shark!
So, now I date my notes too, so I can be aware if it has been awhile since I'd played particular person and to watch them closer for any changes in their play.
I too, like Carbon Poker's player notes features. They help if you want a quick note when your too busy playing one too many tables. It's better than no reminder at all, when you forget to write one before the person is gone.