I pretty much agree with Jon. I have most of my success at cash games, mostly of the 'Zoom' variety so I see a lot of hands. Although I have PT4 I don't usually use a HUD. I always take notes on what I see, whether against me or other players against each other. I use tags like 'Tight', 'Cautious', 'Laggy', 'Bluffy', 'Sticky', 'Crazy', 'Strong', 'Weak', and even 'Stupid', etc. I always note if a guy 3-bets or 4-bets and from what position they do it. A lot of people 3-bet in Zoom, especially from the SB, but not all. I also take note of guys who routinely throw out trial bets, or those who like to trap. It helps to know who has done these things in the past and who hasn't.
I pay attention to hands that I've been bet out of. If I see their cards I make a note of what they were when they played that way. That's something that a HUD won't do for you. The info is there of course, but it's not easy to access quickly on the fly. So when I'm thinking about getting into a hand with someone that I have notes on, I always read the notes first. That then gives me an idea of what kind of range they like to play. If I've seen his cards in the past, that info is like gold. A guy who has played aggressively in the past, but only with really strong hands that I've seen and made note of is completely different from a guy who plays aggressively with pretty much any two.
If I find that my notes are getting too long for a player I try to condense they down using tags so that they're easier to read when the clock is ticking.