I have a boxer mix myself, and have been around many boxers that *are* representative of the breed.
They are ***SMART*** dogs, fun-loving, playful, incredibly energetic, and extremely good with children.
Now for the bad news: They are ***SMART*** dogs, fun-loving, playful, and incredibly energetic. If you don't put the time and energy into training them properly, they will grow up to be distructive, pushy, and agressive. There aren't the kind of dog that you can just leave out in the yard.
If you do put the time and the tons of effort into training them and paying attention to them, you'll get some of the best dogs I've ever met. They pick up new tricks and behaviors suprizingly fast, and actually look forward to being trained. (training = attention and new challenges) They love to play, and will usually out-last most people.

I've also heard them described as natural clowns, and I believe that completely. If they do something, and you laugh, they take that as positive reenforcement and will go out of their way to get you to laugh more.
My dog has learned that if she steals my socks off the bed as I'm getting dressed in the morning, I'll chase her to get them back. So she'll sit by the bed while I lay out my clothes, waiting until I have my back turned, or I'm putting on my shirt, and then will sneak up slowly and try to grab them.

If she gets caught mid-steal, she'll drop them, sit back down, and look around like "I wasn't doing anything! It wasn't me!" and act all innocent.
Boxers are sweet by nature, like playful little kids. You just have to give them attention, and remind them occasionally that YOU are the alpha in the house, and not them. Otherwise you end up with a rotten kid.