I think it just makes the romance bar fill more with each kiss than with a sim who does not have the trait. I only ever buy it to max out charisma quickly.
If I read the tunings correctly, it should:
- Increase the effective charisma level by 3.
- Add 100 to the score of all kissing interactions; unless the sim has a pretty big penalty for his score (such as from being sabotaged), that should guarantee a success.
- Increase charisma gains for kissing interactions.
So, yeah, far less useful after maxing charisma, though it should still be useful in that kisses should almost never fail.