after having completed the game, i can say that it's definitely worthwhile. there's a new gameplay element (which is repeated a hell of a lot), that is immensely fun. you're charged with protecting the little sisters as they harvest - but prior to doing so, you can set up all manor of traps. as soon as they start harvesting, you get bombarded with wave after wave of splicers, which (if you've prepared correctly) get gunned down by bots, fly into the air, attacked by bees, electrocuted, exploded, etc - leaving you to pick off the remainder. very difficult at first, but towards the end when you have more devices at your disposal, it's extremely satisfying.
also, the story, while no masterpiece, verges on "pretty decent for a sequel shooter".