Should you play Thief?
Let me put it this way. I've been playing games steadily since 1993. I've played every kind of game and enjoyed most of them. However, I have never enjoyed a game as much as Thief. Out of the thousands of games I've played it is my favourite. Its so in-depth, so fun, the gameplay is increadible. You may think that the graphics are a bit tried for today's age, but they really do their job for the gameplay. The #1 thing that turns people off from it is its difficulity. But, like most Looking Glass Games, it is increadibly indepth and that makes it naturally hard. I would actually say that Dues Ex is somewhat more difficult, because once you get the hang of thief it gets easier. As for Syberia 2, I simply found it a bit too easy at parts, so...
In any case, Thief - one of the best games ever made... well, at least for me.
k: But if you don't want to, Dues Ex is definatelly a second choice...