Annnnd... Bioshock 2
is now finished ! Took me a little less than 10 hours, which is quite average, I'll say.
So, Bioshock 2 is considered as inferior to Bioshock 1 and now that I've completed both, I know why: there is no surprise. Ypu already know Rapture. Okay, there are lots of new plasmids, new enemies (quite cool ones! The Big Sister is quite a valuable opponent!)... but it's still Rapture like you knew it in #1.
Apart from that, I found it to be a really good game, with an interesting story, excellent graphics and effects, nice soundtrack and absolutely no problem to play it on keyboard/mouse. I thoroughly enjoyed it, even if nothing really surprised me.
So, I'd say that I have the feeling Bioshock 2 is underrated just because there is no element of surprise anymore... But it's still a great game!
My complete list in 2012 : http://www.gog.com/en/forum/general/games_finished_in_2012_1/post15