I haven't used Aquilonian since it's much newer, but I have a few characters with Zingaran. I don't remember all of the rules off the top of my head, but basically, here's what I think:
You never have to spend feat slots on Improved Feint ever again, just pile skill ranks into K:ZFS and get it and other free feats/specials. I think Unwavering Focus is the next best. Free Intricate Swordplay is hot for my Brythunian thiefess who has crummy STR and DEX but buxom CHA. What you won't get out of the skill is direct kill, it's a support mechanism for Sneak Attack. So, all thieves should take it since thieves need Improved Feint (solo work, killing barbarians). In theory, a scholar could use it for stuff like Improved Disarm to annoy enemies while others kill them, but the scholar would need to know how to use a sword (not an issue for groups who always start as a thief at first level but an issue for people who don't want to optimize) and could be doing other stuff like piling into Craft: Alchemy to get damage output.
For any sort of 1h sword SA build, it's easily the best skill in the game as Tumble doesn't save two feat slots. It does make characters more complex, of course, as it expands combat options with stuff you would probably never end up with if you didn't have the skill.
Now, rather than introducing new, must have skills, making existing skills more generally useful, especially profession and perform (the non-ritual ones), would be nice.