Personally, I think Gabriel Knight 3 is one of the best games I've ever played. All of the Gabriel Knights are in fact, and the third is my second favorite. Granted, 3D was in its "growing pains" phase at that point, but I don't think that takes anything away from the fantastic story. In fact, I like what the 3D brings to the world, and how it feels you are actually "there" in a sense.
The controls do have a bit of a learning curve. I think it's easiest to move the camera with the mouse by holding down the cursor and dragging it where you want the character (Gabriel or Grace) to go. Then you just click in the area you've moved the camera, and Gabriel or Grace will automatically walk there. And I love the game's interface. I'd actually like to see more games implement it.
One puzzle in the game is a bit convoluted and doesn't make a whole lot of sense, but another puzzle stands out as being the best puzzle I've experienced in any game.
However, if you haven't played them yet, I suggest you play games 1 and 2 first. That way, you follow the progressive storyline as its meant to be experienced.