First, the spoiler-free version: It's a fantastic movie. Thrilling, funny, and emotional, with great cinematography and brilliantly creative setpieces. I think the visuals and concepts often overwhelm the plot and characters, but that isn't an indictment of the work that anyone did on the film, just a matter of balance that Christopher Nolan didn't quite get right. The basic plotline isn't too difficult to understand thanks to plenty of expositionary dialogue throughout, and Nolan's constantly dropping hints about what the bigger picture might be until he wraps it up with a wonderfully ambiguous ending that invites speculation and discussion.
Now then, regarding that ending, and what it all means... if you haven't seen the film, stop reading now.
No, seriously, stop.
Okay, so: the ending. Is Cobb dreaming? Does the top keep spinning forever? No and no... at least based on the information we get in the movie. If you bring in outside ideas and assumptions, you can use the stuff in the movie to make a ton of arguments, but by looking at the characters and their motivations, there just isn't a sound argument that Dom stays in Limbo forever with the memories of his kids and father-in-law.
But the kids are wearing the same clothes! Yes, they are, but Dom sees their faces. While he was unable to keep projections of his kids out of the various dreamworlds, he never let himself see their faces because he knew that if he rebuilt them from memory, he'd lose sight of the fact that he was in a dream and never come out of it. He wouldn't want to do that since his entire motivation for the entire movie is to get back to his kids.
But he had Mal's totem! She could be playing a trick on him! A trick in which he gets over the guilt of her death and never sees her again? She wouldn't want to do that since her entire motivation for the entire movie is to spend eternity with Dom. I think it's far more likely that Dom tricked Mal since he admitted that he did as much using the totem she had forgotten the use of.
But Saito would've wanted revenge! The top never falls after Dom goes to sleep in Mombasa! True, but how would Saito benefit from permanently neutralizing the one guy who can neutralize Fischer? He wouldn't, and to do so would run counter to his entire motivation for the entire film.
But Joseph Gordon-Levitt looked absurdly handsome! You should kidnap the costume designer and have him redo your wardrobe! Yes. He was and I should.
The Cobol goons are projections of Cobb's subconscious trying to stop him from completing his mission! The narrow alleyway replicates a common dream scenario! If they are, why are they preventing him from carrying out the mission that will reunite him with his kids?
Michael Caine told him to wake up! It's all a scheme by him to get revenge for Mal's death! And spite his grandchildren by depriving them of their father when they've already lost their mother? Caine's motives are the same as Cobb's - even if Caine disapproves of Cobb's methods.
The more I think about it, the more I realize that all the characters in the film have motivations and objectives that sync up with Cobb completing his quest and getting back to his kids, and if they don't directly, then they certainly don't benefit from him ending up a vegetable in Limbo. The only exceptions are Mal and the Cobol goons, but the former is insane and defeated while there simply isn't evidence in the film that the latter has any role in the proceedings. The speculation and what-ifs around this movie are a ton of fun, and Nolan's encouraged them with the ending. At the same time, I think any argument that the whole movie is itself a dream depends on dream logic to make its case, and therefore is kinda self-defeating.