I love the Swashbuckler; I like the idea and I like most of the abilities. But it's always seemed a tad underpowered at the higher levels. As it stands it's pretty much a three level class. So I was looking at it and it seemed like it would make sense for a Swashbuckler to get Skirmish, as the Scout ability since it would give the class a little more appeal beyond 3rd level and becuase the swashbuckler is a mobile combatant. Would this unbalance the class?
I judge everything by comparison with the SRD. Look at the Fighter, and try to match up the Swashbuckler's abilities with the Fighter's bonus feats.
* Swashbuckler loses medium, heavy armor but gets 4 skill points/lvl. Call this a balance (based on the Thug variant Fighter in the SRD).
* Swashbuckler's Dodge feature is equal to the Dodge feat; consider later improvements to be taking Dodge again.
* Once Grace reaches +2, it's equal to Lightning Reflexes.
* Equate insightful strike with Weapon Specialization (although it's potentially much more powerful: a high-Int Swashbuckler gets the equivalent of Greater Weapon Specialization, and a 2-weapon fighter essentially gets specialization in 2 weapons).
* At higher levels, Acrobatic Charge is extremely helpful in a lot of Paizo adventures in which rough terrain is sprinkled about with gleeful abandon; "Lucky" is better than a lot of feats; Weakening Critical is great if you have a high crit-range weapon (which pretty much all swashbucklers do); and by 20th level you've got the equivalent of the Epic Reflexes feat from the Grace ability.
Looking at it this way, it lines up pretty nicely with the Fighter. Of course, there are arguments all over about how lame the Fighter is compared to the other classes, but for every such argument, there's another one about how good they are. Either way you look at it, the Fighter is the core class, so by default it is the standard against which to measure other martial classes.
I'd also strongly recommend adding the "Dead Levels" abilities from the WotC site into the Swashbuckler. They're not overwhelmingly powerful, but they add enough flavor or pizzazz to make you not think twice about sticking with the class into 4th level and beyond.