From the little I've played, I can say the gameplay is competent, for the most part. Stealth is pretty solid, and it's reasonably fun to play. Hiding in shadows works pretty much how you'd expect, though the enemy AI isn't as solid as they claimed and, coupled with the fairly restrictive level design, doesn't offer any sort of emergent gameplay. The experience is pretty linear overall.
Sound design is pretty bad, and it actively detracts from the experience, both breaking immersion and impacting the gameplay. Hopefully it will get patched. I personally find the voice acting pretty amateurish and the writing even worse. Garret's lines feel unnatural and forced, making him feel more like a petulant adolescent than a cynical master-thief.
Visuals are quite beautiful, and run well on a new, middle-spec PC(~40 average FPS with everything maxed out). Textures and models aren't great, but the lighting and post-processing are very good, so overall the game looks great (even if the visual design is fairly bland and unoriginal)
For someone who hasn't played the original games, but still enjoys good stealth gameplay, Thief isn't all that bad. The core gameplay is solid. But you won't get much more out of it. Story is standard and boring, and so are the characters.
However if you have played the original games, the lack of proper story, atmosphere, characters and the comparatively very restrictive gameplay will completely kill the experience for you. This is not a good Thief game.