One of the best cards in the game on Expert. Flashlight is consistently very useful, so much so that I run it in the majority of my level 0 Expert decks.
The power of Flashlight lies in the fact that it can reduce test difficulty to 0, if it targets a 1 or 2 shroud location. That means every token except the autofail will succeed. Even the -8 is no threat! So Flashlight essentially equals three guaranteed clues from low-shroud locations. That's very strong.
The average token in the Expert chaos bag is usually somewhere between a -3 and -4, which means directly investigating a 2-shroud location with an Intellect value of less than 5 tends to go very poorly. Yes, you could just let the party's Seeker handle the low-shroud locations; but it's extremely useful for the non-Seekers to chip in with a few clues. The team can advance the Act deck far faster if the Seeker gets some help. Flashlight is one of the most reliable ways to provide that.
Flashlight does face competition from other cards that can help low-Intellect characters pick up clues, cards like Scene of the Crime or "Look what I found!". But it can get more clues than any one of these events, albeit at the cost of more actions.
In a pinch, of course, Flashlight can also be used on a 3+ shroud location to boost your odds of success. There its benefit is much less dramatic; and on Expert it's generally best to either a) cheat out clues from high-shroud locations or b) let the Seeker handle them. But sometimes you've got to improvise, and if you urgently need to crack a high-shroud location, Flashlight can certainly help.
A further benefit of Flashlight is that it stays in play when its charges are exhausted. This makes it useful fodder to protect your essential assets from the likes of Crypt Chill, Pushed into the Beyond, Corrosion, etc.
Three more synergies should be mentioned. First, Flashlight turns Double or Nothing into a de facto Deduction (if used on a low-shroud location), which is a very nice play. Second, Flashlight works very well in conjunction with "Look what I found!". Investigate a 3 or 4 shroud location with Flashlight and it is guaranteed that you will either succeed, or you will fail by 2 or less and be able to play "Look what I found!". Third, if your Seeker friend is running Arcane Insight, you can combine it with Flashlight to near-automatically crack 3-4 shroud locations, too.
So under what circumstances should a level 0 Expert deck not run Flashlight? The first is if you can't afford to give up a hand slot. This isn't often the case with level 0 decks, but maybe if you were really excited to run Baseball Bat, Flashlight would not work. Of course, XP increases the value of hand slots, and adding powerful two-handed assets like Lightning Gun may necessitate dropping Flashlight down the road.
Seekers can also often skip Flashlight. If you can investigate with a base Intellect of 6+ (as all Seeker builds should aim to do), and your class provides plenty of efficient ways to boost yourself further, Flashlight's ability to auto-succeed on 1-2 shroud locations ceases to be that significant. Flashlight still isn't terrible, as +2 to three investigate tests is a decent return, but it's no longer a game-changer. Seekers can generally find more efficient investigation boosts from their own class, and leave Flashlight to their teammates.