What are boxers like?

I am still grieving from the premature loss of my Ridgeback male, who, though I loved him very much, was ornery around children and dog aggressive. I am looking to get a puppy, and like the looks of boxers, but I don't know much about them. Are they typically aggressive or unmanageable? I've been searching online at backpage.com, and came across some good leads.
   
Any advice on the breed and the online buying process?
   

Oh,doctorevil3232,you are in my house now! My husband and I have had boxers for many years and I can tell you, they are the best breed for families,even with small children. They are very affectionate,very loyal,easy to train and stay puppies for a long time. They are also very protective. They love to have fun and are really clowns. There are some not so great things,as with any breed. Boxers have easily upset stomachs, can be awfully gassy, and are prone to cancer. With a life expectancy of about 10 yrs., they aren't very longlived. We just lost our last two boxers about 7 months ago, and have two boston terriers. They were given to us as "take them or they'll be dumped somewhere" and I love these little dogs,but I miss my boxers. They are all around wonderful dogs.

I've met 5 boxers in my life, and 4 of them live in my neighborhood. Every one of them is/was an absolute sweetheart except one, & that isn't his fault:
   
Casey lives next door, is 2 years old & is almost blind - it's genetic with him, some kinds of cataracts, I think - so he pre-emptively barks at anything that moves because he knows he's at a disadvantage & is vulnerable because he can't see. But he's a nice dog when he's with his person.
   
I knew one years ago, named Babs, who belonged to the guy who staffed the little pet food store I went to. After we were introduced, we played tug-of-war with her rope toy, & she loved it. Every time after that, when I came in, she'd take one look at me, duck behind the counter & grab her rope toy & run for me. I'd drag her around the store, one hand picking out cat food & the other dragging a happily play-growling Babs up & down the aisles. We had a wonderful relationship.
   
They're great dogs.