I use PT4, Find it better than HEM which I have also tried.
The main benefit is being able to identify profitable spots more easily. This is accomplished by both looking at the stats real time. The main stats I use are the normal ones VPIP, PFR and 3-bet. These offer a true reflection of players tendencies with a relatively small sample size. It is worth saying that the stats do not replace player notes but complement them. You can also use them to review hands after the event and find spots where you could have improved your session. This, like posting on a strategy forum, will help you make the right decisions more naturally in game. You can also examine other regs and make notes on them to gain another in game advantage on them.
The benefit is that I am likely to have a pretty decent in game edge against other regs in the game as I have stats (which are fine tuned and not just the standard ones) and at my level the amount of work I do probably beats most of the opposition. It is easy to identify good spots to steal the blinds, 3-bet light, cbet profitable. You still need to be making notes on bet sizing and that kind of thing though in game. It also lets you see if you are playing too loose or tight and see other leaks almost before they happen. Last month I identified, in cash games, I was in too many pots in EP so tightened my ranges in certain positions to counter this. Without Poker Tracker I could not have seen this so easily.
Why did I choose to use it? Why wouldn"t I? Seems like a no brainer.
I recently upgraded PT3 to PT4 and it cost $99, has already paid for itself playing just $10nl.
How am I using the software - See above
Does it make poker more serious and less fun? No - it adds to the game and gives it an extra dimension and helps me make the right decisions. I play for the satisfaction of playing well and gaining extra understanding of the game, not for the money so no it does not make it less fun, the opposite is true if anything although satisfaction would be a better word than fun. When you 4-bet the BB from the button with 97os and know it"s profitable to do it, then you get the spots right time and time again, this gives satisfaction as it feels you have elevated your game to a new level of understanding imo.
When you say identify who I can steal from with 3bets, is it a case of if a stat is above a certain level, then it"s an automatic 3bet for you with any two cards ?
(Without giving away your entire poker strategy, obviously) 
With 3-bets then I would say those situations do obviously exist but people will eventually pick up on this so doing it every round of the table is not going to work that well. You have to look at your own 3-bet stat and your opponents PFR stats and Fold to 3-bet stat. I have seen Brendan make the excellent point on these forums that positional stats are far more accurate than overall stats in this type of spot. There are good spots on some tables where you can steal the blinds literally every round though. You would use the Fold to Steal stats for this.
Just as an example though if you have a player who over a good sample size is opening like 60% of buttons when it folds to him (You use the Att to Steal stat to identify this) and folding to 80% of 3-bets (make sure you get the right stat for this one) then 3-betting ATC will be profitable until he adjusts to you. These spots are more common than you might think.
And a couple of general questions to add to that
Does it not make your game quite robotic ?
How do you find playing live when you don"t have all this information ?
No, not at all it helps. Basically as you are using the stats you are learning new plays and learning to identify the types of player you can make these plays against. It will work very well in a live game and will make your live game better (and yours is already pretty damn good Leigh).
Sorry for the long post, hope it makes sense.
