In my opinion is the best formation...
It's best to have the 3 forwards with a license to roam the pitch, so they can drift all over the pitch in the attacking third as much as they please, like Barcelona do, whilst when the full-backs go forwards, which they should do, they play more like a wing back, or in some extreme cases like daniel Alves will be playing like a winger, these will be your teams wide men, use them to spread the play out and break the defense down by going up and down the wings, recycling the ball, and going down the other wing, the more you do this, the more chance of scoring as you will be stretching the other teams defence out creating mistakes for the forwards to capitalize on.
The two centre backs and one defensive central midfielder will be your back up, and the wing backs should be reasonably forwards so that they can get back in the case of a counter attack.
The two centre mid-fielders jobs are to basically just keep the possession and to keep recycling the ball, giving it to the wings, checking to give the wide men an option back and then to spread it around to the other side, of course while that's going on the 3 forwards will be drifting around looking for space to receive the ball, if a central mid spots one of them in space and can put a good ball to them, they will almost certainly make something productive with only one defender to beat before they are in on goal.
That is the pro of this formation.
While is works brilliantly if you have good central midfield players who can spread the ball around like there is no tomorrow, if your have poor players who can't hit long balls and can't even keep the ball via good control and short passing, then this formation is going to cause your team problems, and although it can be very strong defensively, this formation can tend to get caught out by a superb counter attack, as teams that play this formation like Barcelona, often rely on the offside trap to stop any threat of counters, and when one does get through, unless you have a brilliant keeper like Valdes who can spot a threat a mile off and come forward to snuff the attack out, you are fu**ed basically.
When you have good players, it works brilliantly, possibly the best formation for good teams, however when your players aren't that little bit special, it can bite you in the ass.