pre-class level 7th, there is not much benefit to being a duelist. A regular fighter (hell, a regular swashbuckler) will mop the floor with them. Post-class level 7th, lessee...
Fighting defensively, they can 'double dip' for an AC bonus, capped by class level (and capped for one of the dips for int bonus).
Assume an Int of at least 30 (original 18, inherent, book +1 (27 500), enhancement item +6 (36 000), character level advancement +5) for at least +10 max. bonus for the int part.
So at 10th class level you could get a +20 AC bonus when fighting defensively.
But you can't use armor and shield. So measure the +8 bracers of AC (64 000) armor bonus vs the +5/+5 armor/shield (51 820) bonus you would be giving up, and you are 2 behind. The armor and shield themselves could count at 10 more. So now the duelist has an AC bonus of 8 over the "sword and board" guy, but at a higher and more focused magic item cost that forces your hand on the ability score enhancements (which could have gone into dex for a similar benefit, I suppose) - alternatively, the cost of the book would get much higher if one wants to improve ability scores other than int with one's level based ability score increases. The feats Heavy armor optimization (and greater heavy armor optimization) from Races of Stone would bring this relative bonus of the duelist down to 6. (since the duelist class requires 3 feats that are not dodge, I feel that this is fair - the other person could take dodge, h.a.opt, and g.h.arm.opt)). This bonus would increase by 4 or 8 (depending on whether the armor/shield fighter also has mobility (but that would cost the armor/shield fighter one more feat - on the other hand, fighters are able to get 5 more feats than duelists over 10 class levels, so this may not be impossible). Plus the duelist would not be able to get the "special bonuses" (i.e. those not directly dealing with enhancement) that magical armor and shields can provide.
A duelist who takes a level of wizard could cast shield for a +4 shield bonus, I suppose, to increase the AC some more.
So if you are *very* patient, you can be rewarded with higher AC if you are going for the "you can't hit me" type of character. On the other hand, a fighter could have spent time enhancing str. to 30 instead of int., and so would get +10 to hit and da! Given Combat Expertise, that gives more leeway for the fighter to take his BAB down and bring his AC up. So a armor/shield fighter with int of at least 13 would in that case be better than a duelist, even after they both reach character level 13 (the earliest level a duelist possibly could get to class level 7th). The +10 damage surpasses the duelist's precise strike bonus damage of average 7, and is usable vs. more opponents, such as constructs and the undead.
So the only way I can see the duelist doing well is with that shield spell, which costs another character level. And probably a BAB.
So, given the shield spell, is a net +4 AC (+8 in situations involving mobility) at 14th+ character level under those hyper-tweaked circumstances, worth losing 3 points of damage (10 in situations involving those critters immune to sneak attacks), some feats (or other character class abilities), and the possible "other" bonuses of armor and shield (heavy fortification comes to mind, as does energy resistance, invulnerability, etc.)? I would not think so. And without the shield spell, the duelist merely equals the tweaked out armor/shield AC, but still is behind in damage, non-plus armor bonuses, etc.
Also note, that if the duelist doesn't have time to start fighting defensively, they are at a severe AC disadvantage at the beginning of the combat.
Oh, and we haven't yet looked at the fact that their weapon selection to use that bonus damage is more limited, with less average damage than, say a bastard sword.
I guess this is a long-winded way of saying that, no, under no circumstances is taking class levels in duelist a good idea, mechanics-wise.
