Sixth Sense competes with tried-and-true Rite of Seeking. They are both level 0 Mystic Spell assets that let you investigate with willpower in place of intellect, which can be a crucial ability for low intellect investigators such as Agnes and Akachi. Let's compare these two Spells:
Sixth Sense picks up clues one at a time, whereas Rite of Seeking picks them up in twos. Rite of Seeking is therefore more action efficient at clearing multiple clue locations, which gives it an advantage in high player counts by accelerating your clue gathering. In theory Sixth Sense can create action efficiency by saving actions that would have been spent moving, but obviously saving the skill test on an extra investigate is usually better. When playing solo though, Sixth Sense is just as fast against 1 clue locations.
Sixth Sense costs 3 resources and Rite of Seeking costs 4. Not a major difference but it might be important if you are playing Dark Horse.
Sixth Sense doesn't deplete charges. With Sixth Sense in play, you can investigate using as often as you like.
Sixth Sense rewards you for drawing , , , and tokens where Rite of Seeking punishes you (or at least restricts its use to your last action). This might make Sixth Sense more desirable for Jim Culver who is already encouraged to increase his odds of drawing tokens.
The reward Sixth Sense offers you for drawing a funky token is worth talking about in more detail because there's a lot going on there. It will allow you to sometimes investigate at dangerous or high shroud locations that you might otherwise be locked out of. It can also protect you from token effects, large negative modifiers, and the Haunted keyword by lowering the shroud of a location you are investigating (although you must change the location you are investigating at in order to use this effect).
Unless you're willing to use Dark Prophecy this effect isn't reliable, but it is a kind of insurance against the chaos bag. Finally, a location must be revealed for Sixth Sense to let you investigate at it instead or use its shroud, meaning it pays to follow another player around, investigating to pick up clues left behind with the added possibility that you sometimes get a clue from a location they just revealed instead.
To conclude: Rite of Seeking is still the premier level 0 Spell for picking up lots of clues quickly, but Sixth Sense offers a persistent and highly reliable way to investigate using willpower. Having played a little with it at this point, its definitely a strong card for allowing Mystics to consistently contribute to investigating, so try it out!