My last dog was a pit/boxer mix - and he was one of the sweetest dogs I've ever known. He was extremely loyal - and very easy to train. He would do anything for me, he didn't need treats or toys - he just did it because I asked him to.He was also fairly good sized - he weighed 80# - and there wasn't an ounce of fat on him!
However - you do want to watch their aggressiveness. Because of the pit in them - your dog will have a tendency towards dog aggression. However - if you train them correctly - and socialize them - you can get past the dog aggression.. The downside of their owner loyalty is that they can be a little over protective. Again - this is something you can minimize with training and socializtion.
What age is your dog? You said that you adopted him (or her) - so I'm guessing he/she is already neutered or spayed. Our dog was older (2 1/2) when he was nuetered - so that didn't help his aggressiveness.
All in all - our boxer/pit was a great dog. Even though I only had him for a little over a year - he was the best dog I've ever owned!