Inception, like The Matrix and Waking Life, challenges the viewer's concept of reality. All three films do so by wrapping the philosophy inside a grenade of visual effects. The risk taken by these films is that viewers may not "get it" -- that they will walk away more frustrated than amazed. Of the three, The Matrix is still the benchmark for mass appeal but Inception should be a close second.
The definition of inception is "the beginning of something, a commencement or origin" and for the purposes of this film, it is the seed of an idea that can be planted deep in a person's subconscious through a shared dream state. To believe that this is possible, one must first accept the film's premise that dreams can be not only shared but also built, directed, and manipulated by others. Cobb (Leonardo Di Caprio) and Arthur (Joseph Gordon Levitt) work together with a carefully selected crew of chemists, architects and forgers to infiltrate minds and secure secrets for their clients. We learn that extracting information is much easier than planting an idea and that both activities are not only illegal but also very dangerous.
It's not the first film to illustrate the dangers of messing with people's minds -- both Eternal Sunshine of the Spotless Mind and Being John Malkovich also explore this realm. While the crumbling memories of Eternal Sunshine are well-represented in Inception, I also think writer-director Christopher Nolan picked up on Malkovich's pride of self in building believable worlds for the dreamer.
Nolan wrote the first draft of the script about 8 years ago but has said that the idea grew from his own attempts (and apparent success) of lucid dreaming. Nolan's scripts frequently delve into the mind's corners, dragging the viewer into situations that seem just barely plausible. In lesser films I would be not so forgiving of the apparent lack of internal consistency afforded one key plot point, the kick. I won't discuss it here but for a great summary of my problem with it, check point #39 in "50 Things About Inception" by Adam Best (but be warned, the article is wall-to-wall spoilers).
My only other complaint is that I would have preferred to see better use of some of the supporting cast -- Levitt's Arthur is particularly strong as the straight man who just wants to go in and get the job done but is also able to adapt to Cobb's penchant for bending the rules. On the other hand, if Inception were to ever become a franchise, Nolan could easily develop a film around Arthur or almost any one of the other characters.
While there is grumbling across the net about the ending, I appreciated it, just as I don't mind following Nolan's breadcrumbs through a film. The breadcrumbs are what make Memento and his adaptation of The Prestige so much fun to watch again and again; once you know how each film ends, watching one from the beginning adds a new layer to your understanding. Inception is very similar in its structure and while I can't say I will rush to see it a second time in its first run, I am already looking forward to seeing Cobb and his crew again.
P.S. If you've seen the film, or if you want a better idea of the world in which it takes place, check out Inception: the Cobol Job, a prequel in comic form.