Bluff is a skill reflecting your ability to deceive and lie. It uses the Charisma stat as a modifier
What can you do with Bluff?
Lie: Bluff allows you to have your lies believed. It is opposed by Sense Motive and modified by the believability of the lie and the condition of your opponent.
Feint: You can use bluff in combat to Feint. This allows you to cause your opponent to lose their Dexterity bonus to armor class on your next attack against them. This is a standard action, which means it takes your turn to accomplish. The difficulty is 10+BAB+Wisdom bonus, or 10+Sense Motive whichever is higher. It's more difficult to accomplish against unarmed opponents and non-intelligent opponents. There is a feat which allows you to do this and make an attack in the same turn.
Secret Messages: This allows you to pass secret messages - that is communicate with party members in code in front of other people. It is a DC 15 check for simple messages and a DC 20 check for more complicated ones.
Which of these have ground for use?
What we are talking about here is a mechanism for social conflict. And looking at our desires for what we want social combat to accomplish it is a passable mechanic.
I'd prefer a more entertaining subsystem - perhaps with a greater degree of player skill involved. This reduces lying to an attack roll using charisma as the modifier against a fluctuating AC modified by Wisdom. I think there is something much more entertaining that can be done with this skill.
Perhaps some sort of card mechanic to simulate social resources, or perhaps like in a debate where certain stances are selected and then RPS arguments are made and success is determined by a modified roll. Or something more entertaining then an attack roll that basically comes down to 'pass/fail'
What is it we gain by having this skill?
A simple quick system for answering the question - does the target believe me?
What do we lose?
Some interest and value to be extracted from this conflict. The success of the skill is all or nothing which is very uninteresting, there is no degree of success.
Conclusions & Suggestions:
Bluff is one of the checks that is generally an opposed check, so is particularly susceptible to the high variance of the d20 roll. In defense of bluff, it is a skill where a pretty substantial variance is an accurate representation of the process of deceiving another person.
Usually when constructing one of these situations (NPC's and social conflicts) it is more interesting to create a set of interesting choices for the players to make rather then a straight roll to determine the achievement of the goal.
This is really my core argument against skills handled this way. Let's play a game of 'you win' isn't a very engaging activity. ("Roll a die, if it's over 10 you win!") I am not saying this always happens, but this is a specific case where it can occur frequently. Any decision made or skill that is used turns into a modifier (+/- 5 in this case). And then you win or lose based on the roll. Often Winning means 'bypass having to come up with an actual solution' which is code for 'bypassing playing the game' and losing means 'we're forced to actually play the game'.
This is fundamentally less fulfilling then providing the players with an interesting choice.
But in snap situations like a guard coming around the corner the roll becomes more interesting because the consequence is more interesting (having to engage in combat).