Honestly I find it as the second best of the Bioshock games, with Infinite being the best.
And really, that speaks wonders for what I think about the Bioshock series as Bioshock the original, whilst my least favourite of the games, is one of my favourite games overall still.
Its twist, if it even really has one, is nowhere near as good as 1s or 2s. Its story, IMO, is better paced though, and it focuses more on the personal relationship between the protagonist and his daughter. Its more personal, you see a much better scene based off your choices at the end that IMO is more emotional, and it does just flow, rather than the original Bioshock's which kind of hit a dead point after its big reveal. Additionally, its got some great fan service in there with Mark Meltzer, who was used lots during the advertising of Bioshock through the "There's something in the Sea" website, which I believe no longer exists, only redirects to the main Bioshock 2 site, and that was its own personal story that was kind of sad to follow.
The gameplay though, is a lot better, and that's what makes it a better game IMO. The upgrade system is a lot better, and whilst you still have basically the same arsenal the ability to dual wield things removed a LOT of the clunkiness from Bioshock 1's combat. Also, the Drill. Fucking fun to use. Hacking is a lot better than in 1. No longer pauses the game, but that's not a bad thing, and it takes nowhere near as long or is as drawn out as BS1 hacking. Big Sisters are also fun to fight, and the Little Sister escort parts are actually alright, largely 'cause you're allowed to plan and set up before the attack begins, rather than the crap in BS1.
Is it worth playing?
Depends on what you want to get out of it. Its a great game, and if you like the old Quake/Unreal style shooting in games then it'll have you covered, but if you're looking for some big twist, or a science-based story like Levine does [Genetics in 1, Quantum Mechanics in Infinite, 2 kinda just says "Yeah, that genetic stuff still exists", but focuses on everything else in the story, rather than the science side of it], you're going to be disappointed.