I have a great comedic use for a Succubus:

The players are barricaded inside their HQ castle. Outside there's a couple of vampire spawns that envy the fortress but can't seize it due to the "Forbiddance" weakness.

Then, a sweet Succubus enters the castle using "Etherealness" and tries to stealthily single-out one weak-minded character, charm and convince him to invite her "inlaws" for dinner. 

The vampires would try to keep the charade in front of the charmed player (just for fun) while inventing excuses to leave the dinner table and sneak around the building trying to kill the rest of the party. And if they're found out and destroyed, the Succubus can pretend that she didn't know her "parents" had become vampires (Deception +9) and trick the adventurers into helping her to revive them to their former "parental goodness" (when in reality the vampires were a pair of evil necromancers from the start, even before undeath). Cue, a party with low Insight raising their own enemies... Who might actually come around and become non-hostile to the players, seeing how easy they're to manipulate.

How effective would a Succubus be for a sidekick?

Most of the time, it ends really badly for whoever made the deal. I had a guy make a deal that required his Wisdom saving throw go up by two every time the Succubus tested his will. His character died two days after the deal. Unless you can perfectly create a deal which would most likely involve a high bargain and for the Succubus that won't make her mad or get you killed in the process

Quote from BobbyJackson >>

I have a great comedic use for a Succubus:

The players are barricaded inside their HQ castle. Outside there's a couple of vampire spawns that envy the fortress but can't seize it due to the "Forbiddance" weakness.

Then, a sweet Succubus enters the castle using "Etherealness" and tries to stealthily single-out one weak-minded character, charm and convince him to invite her "inlaws" for dinner. 

The vampires would try to keep the charade in front of the charmed player (just for fun) while inventing excuses to leave the dinner table and sneak around the building trying to kill the rest of the party. And if they're found out and destroyed, the Succubus can pretend that she didn't know her "parents" had become vampires (Deception +9) and trick the adventurers into helping her to revive them to their former "parental goodness" (when in reality the vampires were a pair of evil necromancers from the start, even before undeath). Cue, a party with low Insight raising their own enemies... Who might actually come around and become non-hostile to the players, seeing how easy they're to manipulate.

 This could be a great quest-line for an applicable party.  The Succubus may use a permanent Arcanist’s Magic Aura to hide the fact she’s a Fiend from any troublesome Paladins (conveniently forgetting to share with her allies).

Quote from BuildsmithMaster >>

How effective would a Succubus be for a sidekick?

 Extremely effective.  They have an excellent toolset, beating the Imp and Doppelganger at their own games.  It’s just that they are very rarely trustworthy.

Quote from Dr_Frederick >>

The monster art for this entry is really good, and makes the fiend seem like a cool, dynamic, and formidable opponent.

It’s a shame that most succubi don’t really deserve such cool art.  They just catfish people in the dreamscape, and then make off with their souls and/or money.  When the going gets rough, they just phase out and fly away, rarely being invested enough to put up an actual fight.

I’m going to assume the one in the art is a scrappy exception to the rule.

 I just recalled some lore about Succubi who go out of their way to exclusively devour “beautiful” monsters, like Mariliths and [monsters]Erinyes[/monster].

This means that they launch hunting expeditions targeting monsters three or four times their CR (often occupying influential positions in their own hierarchies). That’s pretty scrappy.

would it be ok to make a succubus/ incubus as a playable character.

that would be cool, i would totally be a playtester

Quote from BuildsmithMaster >>

How effective would a Succubus be for a sidekick?

 Far too effective.  Tasha's says you can only use a creature of CR 1/2 or lower as a sidekick.

Can't wait for my lesbian Paladin to have a gay panic before getting ahold of herself and slamming about 5 Divine Smites into the thot's forehead.

I actually have a succubus serving me as my right hand in a campaign I’m doing which might just be because my character killed several gods and stole there powers.

Quote from TheTriforceKing >>

I actually have a succubus serving me as my right hand in a campaign I’m doing which might just be because my character killed several gods and stole there powers.

 Epic level adventures are fun like that.

Quote from powerspear >>

Can't wait for my lesbian Paladin to have a gay panic before getting ahold of herself and slamming about 5 Divine Smites into the thot's forehead.

 Oh, that’s gold.  Hope she gets ahold of herself early on, getting enthralled into fighting for some vixen is an awful embarrassing way to be outed.

If you ever have an encounter with a succubus, always remember the "S.I.P" maneuver.

S: Stay away.

I: Inform your nearest L/G Paladin.

P: Pray to the gods that it doesn't charm you.

Interesting how these monsters are seen as masters of seduction but they themselves are not immune to being charmed. 

Best creature to have slowly pick off town guards by seducing them and having them dismiss their inferiors as well as using the draining kiss. Then once security is weakened they can begin their fiendish insurrection with no resistence.

I think the idea is that the Succubus charms a powerful creature who does the dirty work for her, and then to tie off loose ends once the damage is done it's an easy way for her to do massive damage to a completely willing creature.

Quote from Herald >>

One would think that the kiss of a Succubus/ Incubus would restore it's own hit points, otherwise why bother?

 I think the idea is that the Succubus charms a powerful creature who does the dirty work for her and then to tie off loose ends once the damage is done it's an easy way for her to do massive damage to a completely willing creature.

honestly, looking at this, it seems like the absolute perfect thing to use on bards or something that try to seduce everything. If they flirt with it, it kisses them and kills them dead. I've calculated the damage, and average damage from two Draining Kiss attacks (one willing, one charmed) can kill a bard with a +1 Constitution modifier up to like, 10th level or so. Plus, bards don't get Wisdom saves and could easily be charmed by it. I really hope that I am never DM for such a player, but if I was, this is how I would deal with it.  

It doesn't even need to worry about sneaking up because of Etherealness 

For the Silver Flame!

Funny that these things aren't immune to charming.