First things first. I've read a bit of discussion online about how this movie is too confusing and hard to follow... thats simply not the came. True there are layers within layers (within layers, etc) to the narrative, but the editing is such that it is very easy to follow. The rules of the game in Inception are established very clearly in the first act and the film keeps to those rules even as the plot moves in increasingly abstract directions.
With that said, despite the intricacies of the plot, this is pretty much a heist film. We meet the team of antiheroes, we become privy to their plan for breaking into an absurdly secure location (in this case, the mind), and then things go horribly awry. The fact that all of this takes place in dreams and that there are extended sequences featuring impossible architecture and Joseph Gorden-Levitt running around on walls and ceilings while battling Cillian Murphy's mental security force is almost secondary (although, these scenes are incredible).
Though there isn't much room for acting amongst all action, I thought the cast did a good job. Though it does help when even minor characters are played by people like Lukas Haas. Leonardo DiCaprio and Marion Cotillard both do a fine job in the lead roles, with Marian Cotillard's Mal appearing simultaneously empathetic and terrifying.
As a cinematic experience, I really enjoyed Inception. The story is interesting, there are surprising twists and turns to the plot, the effects are spectacular while not distracting from the plot, and there are even some funny moments. Most impressive of all, every second of the almost 2 and half hours of running time felt engaging and important. I recommend seeing this in the theater (even the IMax if possible) as this was probably the best cinema experience I've had since the first Matrix.