My experiences playing the new D&D - as well as watching Critical Role online - have shown me that D&D5 still has some rather... interesting "social skills". In my opinion, these can create problems (or at least dilemmas) in terms of how they should be adjudicated at the table.
Here's the rule for "insight":
Insight. Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone's next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.
"Predicting someone's next move" seems fairly interesting and gameable to me - in a tense situation, for instance, taking the time to 'read' someone can be its own risk, and that's interesting. It is also possible to give an interesting and useful answer in most situations without completely tying your hands behind your own back, either as the GM or as a player.
However, most uses of this ability in D&D games seem to be of the former: "searching out a lie".
Most players seem to read this straightforwardly as a "walking lie detector", available to the player/character at all times. Someone is talking to you, and you wonder if they are telling the truth, so you roll for Insight. Roll well enough, and you should be able to know whether they are lying or not.
Hopefully you can see all the problems with this. First of all, unlike other social skills (like Intimidation or Diplomacy or some such), there is no real "fictional trigger" for the roll: the player can just announce that they are doing so, and expect to be able to roll. There is no fictional positioning necessary, unlike something like Intimidation, so it's available at all times. (I suppose you could argue that it can't be used against really bizarre creatures, but certainly it's hard to see why it wouldn't be available for use at any time against your companions, say.)
The scope of the roll is also very unclear, which often leads (in my experience) groups into a pretty terrible habit of rolling for every statement someone makes:
-"I would recommend you take the high pass on the way to Caradhras..."
-"Wait! Is that a lie? I'm rolling an Insight check! Darn, failed."
-"...because the low pass is guarded by a monstrous creature..."
-"Oh, is that
true? I'm rolling again!"
"...hiding at the bottom of a lake near the Gates of Moria..."
-"Oh, that sounds important! I want to make an Insight check, too."
In addition, we have to wonder about whether the roll can be contested when it's PC vs. PC, and that never feels terribly comfortable - at best, it can get in the way of two players enjoying an in-character conversation together.
On top of that, we have to wonder at the meta-information we get from the roll. (For example, if you roll a natural 1, and the GM tells you the character is telling the truth, can you reasonably assume that they are lying?) Rolling in secret can "fix" this problem, but then we run into problems with various D&D rules and abilities, like Luck, which rely on the player knowing the value of the roll.
It's not an easy thing to use, in my opinion, at least not the way it's written in the rules. There are all kinds of other possible issues, as well.
(For instance, it's not entirely obvious how to handle a situation where multiple PCs all want to "get a feel" for the NPC they're talking to. If they all roll, they will almost certainly succeed in every situation. But even if that's OK - perhaps the "walking lie detector team" is net positive to move the game along, after all - how do you handle the differing opinions among the team? Does the GM just dictate what the characters feel and think, and the players must then play that out? Lots more variables here.)
(If your game relies on lies and subterfuge - perhaps some kind of "conspiracy thriller" campaign - then having a "lie detector" ability at all can really be at odds with the very point of play. After all, you want the players to piece together information and figure out which statements are lies by comparing them to what they've learned so far, not by rolling well on a d20 in the very first dialogue. Handling "Insight" in this kind of game is far from obvious. The last time I played D&D 3rd Ed., I was eventually asked to redesign my character, because high social skills made the GM's intended plot and adventure structure more-or-less impossible, since NPCs lying to the PCs were a central feature of the game.)
Are there ways to use this kind of "ability check" which works reliably and leads to good gameplay? Those of you who are playing modern D&D regularly, how do you use this kind of ability in your game?