All right. Let's see... my impressions...
Thematically, it's very interesting. The Ayn-Rand-inspired chief villain is more unique than most villains that come to mind. Being taunted as being a social parasite was much more interesting than being decried as an insignificant hacker by a rogue AI, or a pathetic wimp who doesn't want power by any of a number of cliche villains. I loved seeing that Objectivist sort of character at the forefront; it makes me hope for deeper philosophy in games than trotting out the same old, same old. And Fort Frolic is delightfully creepy.
The morality system choices are pretty much a joke once you realize there's not really a tradeoff for being moral. It's intended to be balanced in gameplay mechanics, so good or evil is a question of which plus is bigger over what the intended effect was meant to be. I get the feeling that it was forced into being a bigger selling point than Ken Levine (a key designer, I don't quite remember his job title atm) meant for it to be. He originally didn't want separate endings to keep the feel a bit less... morally black-and-white, I suppose, but they got forced in anyway.
Game mechanics are good, though there isn't *too* much challenge outside of Big Daddies. And that challenge vanishes when you realize that there is no penalty for death outside of having to walk back from the nearest spawn point.
You buy and upgrade your character through plasmids which can allow for a bit of diversity between characters, but not that huge of an amount. You always can use whatever weapons you have; only plasmids will differ (and you'll get enough ADAM to eventually blur those boundaries to a good extent.)
As for enemies, you have minibosses in the form of Big Daddies, splicers of a few different breeds (~5 or so, though they are pretty distinctive from eachother), steampunk turrets, flying drones and a crappy end boss who really doesn't fit with the rest of the game. I found the limited amount of splicers to be annoying, but not greatly so. Quite a few other games do worse with diversity.
It is a pretty good game IMO. Its most significant problem is the respawn mechanic which removes so much tension. I'm probably in a minority of people who prefer it to System Shock 2.