Quickly, I thought Inception shares a common ancestry with The Matrix (which I found only middling) and many Philip K. Dick novels, most notably Ubik. While I tend not to be impressed with action scenes, and Inception had quite a few, the story and themes in the film were very strong.
In case you haven't seen it, I'll try to talk about those elements without spoiling the movie. The main character, Cobb, is a thief/hacker whose job is to go into people's minds and either steal ideas from them or show them how to stop others from doing the same thing. His particular need is to return to the United States to see his children (I won't go into why he cannot). To do this, he must complete what is essentially a heist (Inception could be called a heist movie, since it involves a complex, Sting-like caper and a gang of experts) in order to do that. Plot complications, driven by Cobb's character flaws, pile up, and the movie builds to its ambiguous denoument with action and metaphysics.
I appreciated that Nolan didn't try to be cryptic out of laziness, which to my mind happens rather frequently in film. He has a carefully laid out story that follows its own internal logic. While adept at action scenes, he does not let them dominate his film (though there are probably more of them than there need to be) but allows the story to unfold.
The character study is strong enough--Leonardo Di Caprio as Cobb plays the role with a brooding intensity that stays on the proper side of mawkishness. Cobb's internal struggle is engaging and the mysterious ending is at once satisfying and unsettling.
It's not a perfect film: as I said, there is a tendency for action sequences to go on a bit long, and aside from Di Caprio, the acting is merely serviceable. Still, the visuals are stunning and add to the depth of the film--this is a case where elaborately constructed shots and special effects are organic to the story, rather than "set pieces" that are shoehorned into the narrative.
Inception is a very good film--to my mind, it ranks among the strongest of science fiction movies.