The charm effect is duplicated by a 1st-level spell plus the additional effects crown of madness offers. Charm person is a better charm effect. Dissonant whispers is a better method of triggering opportunity attacks. Hold person is a better (usually) method of lockdown and damage to the target. The problem is that's 3 spells to do the same thing I can do with a single spell.
I would argue that the spells
Crown is being compared to here are far more universally applicable and less situational the
Crown, in addition to two of them being 1st level spells compared to
Crown being 2nd level. That makes a difference. I would also posit that it is rare that you gain all of the above benefits on a single casting of
Crown, especially with all the 'only maintain it for one round' suggestions.
That doesn't require DM cooperation in the slightest. It requires making use of the spell as is, and that can be helped by player teamwork with other options such as effect that hold targets in place or move targets between their turns.
I think it does require significant DM cooperation to have all the stars align to get the max benefit from the spell here described, not to mention not just playing the NPCs such to minimize the effectiveness of the spell once cast. Some might argue that is just poor DMing and NPCs should be played according to their type, traits and personality, but it is a temptation I have seen quite a bit over time though.
It does all depend upon what the DM interprets the charm to involve. For some DMs, it doesn't do anything other than the explicit items under charm in conditions. For others, you get a friendly attitude built in as well.
While most things in 5e are up for DM interpretation, I don't see any particular need for it with the charmed condition: the target cannot harm or or target the charmer with harmful effects; and the charmer has advantage on ability checks to interact with the target socially. The 'friendly acquaintance' verbiage is unique to the Charm Person spell.
You don't just prevent the enemy from using his action, you make him use it to aid your party and if you are engaged in combat with him you force him to choose between taking an OA on his turn or attacking his freind again next turn. He can't disengage because he has already used his action.
There are a lot of assumptions present in this situation. I'm not saying that this does not happen, or even that it might not happen semi regularly. But it is not an assured thing. You have to have a target next to an ally that is also engaged with your party. So far, a fairly common occurrence. Then, it must be worth your action, concentration and a second level spell slot to have the target potentially spend their action to to make an attack against their companion. Even then, the target can move away from an ally without taking an Opportunity Attack by just moving to the side without moving out of reach of a foe. Or the ally could be the one moving away from the crazy guy with the glowing crown, perhaps in a similar manner if Opportunity Attacks are an issue. Finally, would eating an OA (which is not guaranteed to hit) worth the cost of the spell and action used to cast it? Maybe.
It is a very thoughtful and tactical approach to the game, and all have provided good points on how to use the spell well. Some may not want to get into that detail and just want a more direct effect though.