Well as controversial as this particular subject may be, I would have to agree that sometimes you should act on impluse and other times you should think things through.
Mainly, i would have to say it depends on how serious the concequinces and/or the situation may be.
Some Examples:

~:IMPLUSE:~
You want that New CD. You pocket it and walk out the door. You will sound the alarm because you forgot to remove the alarm sensor. Busted. Depending on age, Jail time or juvie. All because you acted on impluse rather then thinking out what you should do and how to go about getting what you desire.

~:THINKING FIRST:~
You want that brand new Cd. You don't just pocket it but rather you walk around a while "thinking" the best way to pull this off. You remember the last cd you bought had a tiny metal strip enclosed within white plastic in your last purchased cd. You take the cd where you wont be seen and pocket it but instead of just running out the door, you use the restroom and make sure you wont sound the alarm as you exit. Boom, you have that new CD.

As i said earlier though, its best to balance the two out. Sometimes acting on impulse is good other times its not. I guess thinking would be the best option as you always are aware of the possible side affects of your actions. But, impulse could have no harmfull or "bad" side affects. Which brings me to my conclusion.
Depends on the situation.