I finished it a few hours ago and I ave to say I loved every second of it. It's very hard to put this above the first game even though in a lot of ways it's superior. The games' length is perfect, there were absolutely no weak spots in the levels, the endgame was great (this is where Bioshock 1 really failed). The main story was definitely weaker and it's also more of the same and it's exactly that aspect that makes Bioshock the standout title of the series.
You really can't ignore the fact that Ken Levine and co. put together Rapture and all its secrets and background stories etc. and the fact that this game basically takes that and makes a few new levels out of it, as good as they are. Any sequel that does this even if it improves upon the original, doesn't deserve to be recognised as the better game, even though objectively speaking, I really do think Bioshock 2 is a better game in most ways.
But please, PLEASE, don't make a Bioshock 3. Have the balls to leave it at this and maybe use Rapture for a different game altogether. I know I sound like a broken record, but make it a different genre with completely different characters and story and have it be set in Rapture in its glory days. My vote goes to a murder-mystery adventure game.