The new Thief game is a mixed bag of tricks. The stealth and gameplay mechanics are fun and compelling, the upgrade system has some interesting choices for you to make in the early game, prioritizing life expectancy over greater stealth or focus abilities. But it has its flaws as well; the sound system is a bit buggy, sometimes leading to the same "background" dialogue from people in the street layering over itself like an echo, and the music sometimes bursts up loudly like you're in the middle of combat when it was just a close stealth mishap.
But if you look over the mostly-cosmetic bugs (I haven't hit any game-breaking bugs in my 1.5 play-throughs of the game yet) it's an entertaining first-person stealth title. If you come at this game expecting to pull out crazy ninja moves and kill every enemy in sight, you need to find another game. That was never the spirit of Thief, and if you try and play a stealth game like Rambo, you shouldn't be surprised that you won't have fun.
With the Challenge mode offering some fun minigames in controlled maps with leaderboards to compete against, and a Custom difficulty mode that lets you toggle over a dozen different options like Iron Man mode (death = restart game) No bow reticle, no Focus, slower movement speed, no fatalities, no being spotted, and more... you can make the game as tough as you want it to be.
I've already begun a Twitch.tv broadcast of a playthrough with Custom difficulty, if you'd care to see how difficult the game is when you're trying not to ever be detected or touch a hair on a guard's head I'm registered there (and on Xbox) as Wizminkey. I've seen a lot of other players sprinting around everywhere trying to turn it into a fast-paced action game for their viewers, and can't help but wonder if those people aren't the loudest naysayers for this game, because I think it's brilliant.