Is Defiance worth it?
I've seen it played on console. It looked like a fairly typical shooter. Had some mixes of Mass Effect (without the cover) and Borderlands-style stuff, though there didn't seem to be as much crap loot as BL throws at you. I have no idea how the PC port of it is, and I also don't watch the show, so I can't be of more help. If you like shooter MMO's it's probably decent.
PC port is fine.
It's a pretty generic game. Get loot, sell most of it to improve the bits you care about.
There are quests to do alone or in groups. There's a storyline or something.
Mini-games to do around the world, like racing and horde mode.
Random nearby event beacons for things like saving engineers from mutants and destroying a bug colony.
AFAIK all enemies scale to the total level of players involved, so doing something alone is just as hard as doing it in an uncoordinated group.
And then there are the arkfalls, which alternates shooting at a rock and shooting at bugs.
I played it on the free weekend for, like, 6 hours straight. Felt like I did half of what was worth doing in the game. If you like a 3rd person quest- and event-driven MMO shooter and like Defiance, go for it. If you only like one or the other, maybe.
EDIT: The small inventory, slow levelup, and P2W undertones are a bit ominous for a game you've already payed for.