I enjoyed it and don't really remember having a problem with the ending, though I vaguely remember that it wasn't the standard happily-ever-after scene. Hopefully the sequel builds on it, rather than resetting the story.

I also saw it at the movies last year, and liked it. so when it came out in BluRay this year I rented it for my folks to see, I had thought it had the right amount of sweetness and action for them to both like. And after I ended, my mom's look at me and asked me "That's it? what happened to the kid and the robot?" which is the first time I noticed that the film had ended a little short on closure (which actually was fine by me), so I speculated (for her sake), that since the movie had a certain Rocky-feeling, with the main boxer not winning the final fight, but actually ending up a crowd favorite, that I could imagine Atom going on to have a successful career, and probably earn the championship on his next chance.. and based on the aunt assisting the last fight (sign of approval), I could imagine the kid will remain with the Aunt, but also keep in close contact with the Dad, and maybe assisting some fights too. Or maybe viceversa, since part of the robot's appeal was the kid that made him dance.
Anyways, since the sequel is just waiting for Hugh Jackman’s agenda to clear up, I guess we will find out what the writer has in mind in regards to what happened after the fight.