[This article was originaly posted to the rec.games.frp.gurps in early October 1996. Since it is so elaborate and detailed, I decided to preserve it for the future by including it here, together with an objection and Thomas' answer to it. I edited a couple of typos and commented a little. -- Incanus]
In the description of this advantage [Common Sense, page B20], it says "This advantage allows an impulsive player to play the part of a thoughtful character".
However, if you aren't an impulsive player Common Sense is a waste of points. IMO, the 10 points is better spent on some other advantage like Combat Reflexes or a couple of levels of Strong Will. Or just save the points to pay for Danger Sense or Empathy. That's how I play the Common Sense advantage.
To be fair, I've changed the description of the advantage to actually make it worthwhile:
You have +4 to IQ to figure out "the right thing to do" in a given situation based on the facts you have at hand. If the character makes his roll vs. IQ +4 the GM must give the player his best assessment of what the character ought to do given the facts the character knows. If the GM feels that it is important that a character do something (or not do something) then he can roll vs.. the character's IQ+4. If the roll is successful, the GM can tell the character's player "what common sense would dictate" in a given situation.
Characters with Common Sense also get +1 to Will to resist their mental disadvantages when they know that it would be socially disruptive or dangerous.
For example, a character with Common Sense and the Lechery disadvantage would get +1 to his Will to avoid making a pass at the Queen or his best friend's wife. He would also get a +1 to Will to avoid following an attractive but fey lady down a dark alley in the rough section of town. He wouldn't get the +1 to Will to avoid dalliances in other situations. (However, you can be damned sure that he will be careful to take precautions to avoid disease and pregnancy, or at least cover his tracks.)
In some situations where the character has a chance to demonstrate his wisdom, the GM can give the character a free +1 Reputation for Prudent Judgment among the people who know him. (This is especially appropriate if the character doesn't have any Will-dependent mental disadvantages.) The character will be sought out as an investigator, arbitrator, and/or judge by his friends and associates.
In my revised advantage the character gets a +1 to Will to resist disadvantage in situations where he knows it would be especially stupid or disruptive to give into his bad habits. (He doesn't get the bonus in other situations if he isn't aware of the danger or the consequences don't seem like that big a deal.) If Strong Will costs +4 points per level and you count the "Must be aware that there will be major repercussions" as a -25% limitation, that's 3 points.
The other effect of my advantage is that the character gets a free +1 Reputation if his Common Sense becomes known. However, this is a 5 point advantage with the limitations "Everyone who knows the character", and "Limited Area of Effect" (since the character only gets the Reputation for Prudence, not anything else). I count "Everyone who knows the character" as "Large Group" (actually, a small group regularly encountered by the character...) which reduces the cost by 25%. I count the "Limited Areas of Effect" as a -25% limitation. This halves the cost of the advantage to a big 2.5 points.
That means the character is paying 10 points for 5.5 points in advantages. This means that you need a reasonably hefty "special effect" to make the advantage worthwhile. Since the IQ roll is basically a very weak version of Empathy [Thomas probably meant Intuition. -- Incanus] (the GM tells you what he think is a prudent course of action based on what your character knows -- which could be completely wrong -- if you make your IQ roll), I decided on a +4 bonus.
This last decision wasn't scientific, it was my gut feeling as a reasonably experienced player/GM about what would make Common Sense a really good advantage to have as opposed to a so-so advantage. IMO, a +2 IQ roll makes it so-so, and a straight IQ roll puts it back in the land of useless advantages.
With a +4 bonus even a relatively dense character has a fairly good chance to "do the right thing" and a smart character will almost always choose the prudent course (barring a critical failure). That's the way I see Common Sense working. If Common Sense doesn't work most of the time, then you don't have Common Sense.
Of course, since this is a purely unofficial variant, you can modify the IQ roll as you like. I also see no reason why you couldn't top the roll out at 14, like you do when rolling against Will-based disadvantages.
In any game I'd run, I'd be sure to let a character's Common Sense trip him up at least once, if the player insisted on using it as a substitute for good play. Remember that Common Sense is based on the character's perceptions which might not be the truth of the situation.