Christopher Nolan is one of a few directors working today whose work is interesting enough that I'll pretty much always see it, just because it's his, regardless of any other factors. Inception, then, is a given.
There are a few things you can always expect from a Nolan film: eye candy, brain candy, and excellent casting. In general, Inception delivers on all three of those points, with my favorite being the casting (as usual). The eye candy was very good, and a lot of it is clearly practical, which makes my heart go pitter-patter, because I adore it when directors make a conscious choice to avoid CGI where possible.
As far as the brain-candy goes, the film isn't nearly as complicated as it might seem at first blush (or, indeed, as some critics are making it out to be). There are four different dream levels that the audience is expected to keep track of (and I have to praise Nolan for expecting things of his audience at all, in an entertainment industry which is constantly catering to the lowest common denominator). But it's not a difficult movie, either.
In retrospect, I'd say it strikes a pretty good balance of "summer popcorn fare" and "high-concept, thinking-man fare".
The one main criticism that I have to make of it, though, is that there might be a little too much narration. I think Nolan was a bit more careful than he was in, say, Memento or The Prestige - he takes a lot of time explaining when he could simply show us stuff. And that failure leads directly to a lack of emotional connection for me with the main character, Dom Cobb (Leonardo DiCaprio), and his dead wife, Mal (Marion Cotillard). What should be something which is beautiful and moving and sad-making left me feeling relatively cold, and I was unable to become emotionally invested in it.
But that was not at all the fault of the actors. If there is one thing Nolan always gets right, it's his casting, and Inception is no exception. The entire cast is wonderful, and they have great chemistry together. The stand-out performance for me was Joseph Gordon-Levitt as Arthur, the point-man, and the straightest straight man to ever straighten his tie and slick back his hair. The control, the subtle looks and small smiles are just perfection, and his physicality is undeniable. The guy is clearly a ninja. My other faves were Ellen Page as Ariadne, the new kid on the block and the architect, the one responsible for the dream world itself; and Tom Hardy as Eames, the forger, witty and British and constantly flirting with Arthur (when Arthur wasn't flirting with Ariadne).
I was able to see most of the "plot twists" coming from a mile away (anyone who doesn't think the ending is going to be ambiguous has no idea who Christopher Nolan is), but that didn't detract at all from my enjoyment of the film. What I want is not so much to be surprised by the crazy plot as to be engrossed in it and impressed by the structure and craftsmanship that went into building it. So the final shot, while completely expected on my part, still made me gleeful, because it was pretty daring for a summer tentpole flick.
There's something very satisfying in a film which is both accessible and smart.