Played all three games - well, I never finished Post Mortem because I encountered a very nasty bug and would have had to start all over again; didn't feel like doing so. Despite its flaws, I liked the game for its atmosphere and story but you need a lot of patience to go through it because of the bugs. So I cannot really recommend it even though from what I've seen it is a good game.
As for Culpa Innata, this is a game that you either love or hate; I don't think there's much inbetween. It has great potential though the ending leaves some questions unanswered. There's a lot of talking, puzzles are ok - some are pretty hard to solve, though. I think the strongest point is probably the world created in the game. The developers paid a lot of attention to detail so that world really feels alive. Also, the storyline is non-linear; you can either follow the sub-plot or concentrate on the main storyline. Either way, you'll be able to finish the game.
Syberia is just beautiful. It takes a while before it really sucks you in but when it does, you'll find it hard to switch off your computer before you've finished the game. The main character is instantly likeable and changes over the course of the game; the locations are beautifully rendered and stunning to look at. Also, both games feature the most touching ending I've ever encountered in a computer game. So to cut it short, I'd recommend Syberia as well
