I worked up a 4.2 standing with them on one of my alts (enough to run level 3 agents).... and as far as I can tell, they give bog standard mission goods. Empire agents give rocket fuel, food and livestock, and 0.0 agents give slaves, dancers and drugs. The mission reward cash is about the same, only select missions include rats with bounty's (although you can pawn the rat tags for bounty), and the loot is just as bad. LP rewards are slightly higher (being as its 0.0) and I assume the faction ship offers are the special Pirate Faction Ships as opposed to the navy ship variants (not sure on the names, sry).
It's just fun fighting a different faction for a change
. That and dodging BoB