If you don't have plans to see this movie, you can check the spoilers here and then come back.
To me, Inception was, at its heart, a really good heist movie; the science fiction and higher concepts trappings built around that central caper for the most part helped though in a few cases hindered the guts of the film. The idea of using dreams as the backdrop for a story like this is definitely unique and Christopher Nolan does a great job coming to the table with a complex and smart idea of how it all works. The world in which the story takes place does require a bit more exposition than I generally like to see in a movie, but for the most part the actors did a good job not making it too clunky and I think it was a good choice to have the idea of traveling through dreams be one the general populace was already familiar with rather than having one character be discovering it entirely out of nowhere. Nolan and Leonardo DiCaprio collaborated nicely to concoct a strong protagonist in Cobb, and I felt his personal stake was well-realized and helped give weight to the plot; I did find DiCaprio's performance a bit too brooding and maudlin at times, but for the most part it was strong. I could harp on the fact that I figured out the bulk of the mysteries revolving around Cobb well before they were revealed (and seriously, I'm not that clever), but upon reflection I think I prefer that to when a filmmaker tries too hard to make their twists come out of nowhere and succeeds by having them make no sense. I also thought Marion Cotillard was particularly haunting as Cobb's lost love, playing equal parts alluring and terrifying, plus riding the wave between flirtation and insanity perfectly. I almost felt bad for Ellen Page in the sense that this was another instance in which she was excellent but I can't deny that her youthful looks take me out of the scene for at least a moment, particularly when she's interacting with Cobb, which almost seems unfair; she can more than hold her own against any other member of this cast when it comes to acting, but unfortunately she can't help her genetics. Ditto to a lesser extent for Ken Watanabe, whom I love as an actor, and his difficulties at times with English. Obviously neither factor was a huge problem, but it frustrated me because I like them both. Joseph Gordon-Levitt got a nice suave sidekick role and also the best action sequence of the whole movie (and there were some real good ones) plus he handles the exposition stuff better than anybody. Tom Hardy was welcome comic relief and quite quick-witted about it. Frankly, Cillian Murphy was window dressing, which is a shame. The movie clocked in at over two and a half hours, but I was enveloped enough in both the plot and action that I could have definitely hung in for another thirty minutes easy. I do think something that will work against Inception is that the advance bill sells it as a bit more revolutionary than it might be, particularly as the much-ballyhooed special effects for creating the dream world are sprinkled in extremely sparingly, but if you don't go in with those expectations, you'll be rewarded with a fun, engrossing film.