I've played through both games and was thoroughly entertained, I love adventure games. The environment is great, you can tell they learned a lot from the prince of persia games, then mastered and refined it for assassin's creed, keeping the game just realistic enough without making it too boring (though being able to take on 10+ guards at once may be a stretch).
While the action part is fun, I have mixed feelings about the plot itself. It is intriguing, but at the end of the second game, I have increasing fears that it could become too over the top or far fetched (but I know it's sci-fi, so it always runs that risk). If you haven't played to the end I don't wanna spoil it, but if you ever get the games and beat it you'll see what I mean.