Here you go, Khadgar.
Succubi are 6 HD demons with a wide array of powers. Their base chassis is quite nice, with a huge charisma bonus and various bonuses to all other ability scores. DR, SR, immunities and telepathy are all good to have too, of course.
Energy Drain is very flavorful, but not too strong. After all, if you're in a position to plant a kiss on someone, you're also in a position to introduce some knifes to their pancreas. The spell-like abilities are all useful to some degree, but will most likely only be used in social or scouting situations.
Furthermore, succubi have a small chance of summoning a vrock each day. If your primary combat ability is a pair of claws, then a disposable melee summon can certainly be useful.
I already touched on this in the previous paragraph, but succubi lack one thing: combat ability. In a game where some, if not most of all encounters will involve combat, this is bad. Summoning a vrock is only useful for an hour every few days: Charm Monster will only delay fights, not end them, and Energy Drain requires either a noncombat situation or grappling (which succubi don't exactly excel at).
Then again, when assigning LA's I tend to assume the player using the monster is doing so in a game where said monster will be useful. No one is going to play a bulette in an urban intrigue game, so I'm not deducting points for the bulette's lack of social skills. Similarly, the succubus doesn't deserve a LA deduction for failing to be useful in-combat: if someone plays it in a combat-heavy campaign it's their own fault.
In the end, I think a level adjustment of +2 would fit best, though I wonder if I should increase it to +3. As always, the hivemind is welcome to opine about this.