One of the appeals of Roleplaying has always been the ability to play something you are not, the true fantasy element of the genre. Just as weak people get to play strong, hulking characters, shy people often wish to play the smooth talking charismatic character.
To that end, many rp systems have attempted social mechanics or "social combat" systems, a way to augment roleplaying with more detailed mechanics so that a person who is not good at talking can still be "diplomatic and smooth talking" in game.
In my 10+ years of RP experience, though I really appreciate the sentiment, I have found that social systems like these simply fall flat. For all the good they attempt to do, what I have seen at the table is that they deaden roleplay. Players get into a different mindset when they are thinking mechanics, and that mindset detracts from good smooth roleplaying. For every person who is bolstered by the system, I find the rest of the party's roleplay declines.
At the end of it all, the simple "persuasion" check at the end of the roleplay conversation seems the best compromise. It at least adds a little mechanical arbitration to compensate for a person who has trouble with roleplay, but it keeps the mechanics more out of mind to allow roleplay to commence. Anything more complicated than that seems to do more harm than good in the long run.
So what are your thoughts? Have you found any social combat or social mechanics that you think were a true boon to the RP experience?