Machinarium is hands-down, the best game I have played in a decade. I just finished it a couple days ago, and I am seriously jonesing for more of the same -- I'd already played Samorost 1/2. This game is stunningly GORGEOUS. The art is meticulously done, the characters are insanely cute, and everything about the game is pure eye-candy. However, the only thing perhaps better than the art is the soundtrack, which I find unequaled in any other game I have ever played. Even the little "jazz trio" in the middle of the game plays a perfect notion of "robot jazz": Not swung, yet also not dry. Although the soundtrack is the perfect atmospheric accompaniment to Machinarium, the fact that an "improved" version is sold with the game is a testament to its high quality.
The puzzles are complex, but not impossibly so, as long as you have enough patience to truly immerse yrself in them. In terms of difficulty, I'd say that they're a bit easier than the puzzles in the original Myst or Riven. I enjoyed Machinarium more than Myst in part because I could survey each scene without losing my orientation in first-person navigation.
I have a couple of extremely minor quibbles with gameplay: Dragging the robot's head up and down gets tiresome on the mouse-finger, and there is actually one dead-end in the game (so be sure to save!) It would also be nice to have the ability to transport saved games between computers. These points are very minor, though, and shouldn't deter anyone.
In sum: The puzzles are very good if you are a patient person, and the music and artwork are second to none.