I've been looking at the Friends cantrip, and the more I think about it the less useful it appears.
It appears to be self-defeating: you can gain advantage in a conversation, but they'll know you manipulated them pretty quickly. You might use it to bypass a guard, but they're all too likely to shout “Intruder!” once it wears off. You could use it on a merchant, but they're all too likely to turn adversarial afterwards, leaving you disadvantaged for the entire duration of your stay in town.
You can't use it to help with someone actively hostile to you, such as during an interrogation, eliminating the kinds of uses where it doesn't matter if they catch on after.
So what exactly is Friends for?
Best Answer
Friends is good for problems that can't be solved by killing people. For example, if you need to get through a gate, and the guard knows the password, killing him won't get you that password. Casting Friends on him, persuading him to tell you the password, and then killing him is a much better plan.
"But you could just force him to tell you the password!" Well, possibly. Loyalty comes into play here, and it's important to remember that the DM has every motivation to make his guards fanatically loyal.
In general, Friends is primarily useful to help you get information. Yes, you'll have to deal with the person afterwards, but you would have had to do that if you'd gotten the information by threatening to kill them. This way, they should give you correct information; getting information by threatening them is a great way to get information that leads to your death.
A secondary use is getting people to do things they wouldn't normally do. This sounds obvious, but (to stay consistent), imagine we have another gate, only this one has a guard behind it. He won't open the gate for you normally, but with the help of Friends you might be able to talk him into it. Obviously, once you're inside you'll probably have to kill him.