For anyone reading this thread, here's a tip; take a look at the number of posts from various people in this thread and compare the number of posts from sly and the thought that he's put into his post and compare with everyone else. That should give you a hint as to where to look for the best advice.
I'll add a couple things of my own; I used to have PokerTracker and switched to HoldEm Manager (HEM) when PT3 came out full of bugs and performance issues. I've never considered going back, and I think HEM has the bulk of the market these days.
For a beginner who is playing only one table I don't think PokerTracker or HEM is needed; just watch your opponents, but, pull up the hand history to see the mucked cards. PT and HEM with display these automatically for you after the hand so if you don't use these programs, pull up the hand history yourself to see. It was amazing to me when Stars came out with their replayer and people didn't even realize the mucked cards had always been available to see.
Either way, make notes on the way player play their
hands. Then when you sit at a table with them again 3 days later you've got that information right away.
Once you start to get into playing multiple tables HEM or PT3 is a must-have, simply because you'll miss action, and even if you see the action, when you're playing 4 or more tables you won't remember who does what most often on which table.
If you don't use PT3/HEM you also won't understand in hand analysis areas what people mean when they say that their opponent was 27/9 which says a lot about how the opponent plays.
One of the best online players, BalugaWhale, doesn't use PT3 or HEM, but, he plays at such high buyin levels that the competition is very narrow so he has played every opponent in thousands of hands and doesn't feel the need to use it himself. Still, he understands what the stats mean so he's able to coach players who do use the programs.