The quick version: It's directed by Christopher Nolan (Memento, Dark Knight), it's big budget and feels as if having created enormous revenues for the studio with Batman, the studio then gave him completely free reign to do whatever with big cash backing.
It's about dreams, and espionage, and personal relationships. I can't say more without spoilers.
The cinematography is excellent. The visuals are full of interesting colours and textures with exquisite framing. Special effects are good, apparently Nolan minimized the use of cgi and used mechanical effects wherever possible, and it shows.
Interestingly, Nolan rejected 3D, as he dislikes the dark effect and didn't want distractions. He believes a film can give a more immersive experience in IMAX rather than 3D. I'm intrigued and would be interested in an IMAX expedition....
It's been compared with various films. For me, the obvious one, that few mention, is the first Matrix... not Bourne, or Memento. But there bits of Bourne too...
While Inception is probably overrated -- top 10 film of all time on IMDB -- it's a very very good film that I plan to watch again in the near future.
Other thought - You should spend money on Inception. As Roger highlighted, there have been very few highly watched films in the last decade that have original IP. Virtually all the top films at the box office have been sequels or based on books, or are re-makes. Pixar films are the main exception, until now. Inception is another rare exception and as such we should support it so people make more original films. Whether you watch Inception at the cinema or buy the blu ray or dvd, I think this is worthy of your cash.