Top Us Off
Let's be real; while there are a lot of deep, interesting things going on in the movie, what people care most about is the ending. That ending.
It's pretty apt that the movie concerned with architectural mazes is itself a puzzle that people are desperate to put together. There's no shortage of theories out there. In fact, it's hard to search Inception on the internet without getting bogged down with "ending this" and "ending that."
Here's our own Shmoop take on what went down… but the rest is up to you.
Let's start with the most important bit of information: the top. Despite some fan theories that the top doesn't work for Cobb because it belonged to Mal, we're going to assume that the top does in fact keep track of reality for Cobb. Totems aren't magically enchanted items; it's simply a matter of knowing how they behave in the real world vs. the dream world.
While letting someone else know your totem is dangerous because they could replicate it, that doesn't mean that Mal once owning the top invalidates Cobb's usage of it. Regardless, we don't see the top fall. Yes, there's a nice ambiguous wobble at the end, but ultimately it's only ambiguous.
Actually that's the point. The whole movie Cobb is obsessed with the top, obsessed with whether or not he is dreaming. This is perfectly exemplified when he wakes from dreams of Mal in Yusuf's basement and splashes himself with water as if to clear his thoughts (or wake himself up?) and then frantically spins the top, which falls to the ground.
The Cobb in the final scene is a changed man, though. He has let go of Mal and overcome his addiction. He no longer needs to obsess over the top so he spins it and then rushes toward his kids.
Rounding it Out
Right, that's all fine and dandy but we still want to know if he is dreaming or not. Come on Nolan, throw us a bone!
Well, he may have done just that. If you ever watch Inception again (let's be honest, you're going to) pay attention to Cobb's wedding ring. Whenever Cobb is in a dream, he's wearing a wedding ring. This makes sense because Mal is alive in his dreams. He is using dreaming as a way to relive the past, a past in which he and Mal were married. However, in the real world he does away with the ring because Mal is no longer with him.
There's the distinction… but where's the evidence?
We don't see a whole lot of Cobb's left hand, but look closely at the customs scene when Cobb hands his passport to the security guy who scrutinizes him. Cobb hands the passport with his left hand and for the briefest of moments we can see—wait for it—no ring!
Take from that what you will; maybe the entire thing is a dream and he just doesn't wear the ring on the first level. Who knows. You can believe anything you'd like, but the ring (and lack thereof) is about the most conclusive evidence you're going to get.