I believe the ending is up to the audience, you can believe he is still in limbo or he (Cobb) actually made it out with Saito.There are so many clues throughout the movie. When Cobb is in the dream world he has his wedding ring on, but in the real world he does not. At the end Cobb is not wearing his ring, which could mean 2 things.
(My observation)
1. He is back in the real world. OR
2. He is still in limbo but the ring disappears because he has encountered Mal by letting her go and by accepting the limbo as reality.
Up to you to choose.
(Researched)
Another clue is the cast overview. In the end Cobb finally reunites with his kids.
1. Kids are wearing different clothes which means Cobb might not be in the dream world as his projection of the kids were always the same.
2. The director of Inception used 2 boy actors and 2 girl actors to play as James and Phillipa(Cobb's children) One boy was 20 months old and the other boy was 2 or 3 years old. One girl was 3 years old and the other girl 5 years old.The boy that was 20 months old played as Cobb's projected son (the past). But in the end the 2-3 year old boy plays as Cobb's son and the same with the girl actors. uhhh who cares? right? Well, this mean that time has passed and that Cobb is possibly in the real world.
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3. Others suggest that people can project age, as Cobb was talking to Mal, saying they did grow old together. And they show you a scene of two old people holding hands. That was a dream, a limbo, so Cobb probably projected his kids as being older and with different clothes.
In the end Cobb spins his totem to see if he is actually in the real world. But as soon as he sees his kids he does not even check if the totem is still spinning. In one scene Cobb has a gun in hand and spins the totem. You can tell he is ready to pull the trigger then and there if it kept spinning, thinking if it all was a dream and thinking if Mal was right. Which can mean he has dropped his philosophy of what's real and what's not, and he now accepts which ever world he is in since he is with his family.
These are not my theories, as various websites posted these theories, weighing in every viable option. These are not ALL the theories to the ending of inception, as there many others out there. I usually think pessimistically (opposite of optimistic) but I want to believe that the totem does drop and that Cobb has finally made it back to his family, completed his mission and is in the real world.
This was a great movie and it was such a great movie I made this long ass post just to give an opinion... Hope I contributed some!
-Watch Shutter Island, as it also has a Inception like ending-
Excuse my grammar, it's a post on a forum, not an AP Lit paper so screw REVISE&EDITING