Sherry's absolutely right about behavior and training issues.
I bought a bull terrier pup (think Spuds McKenzie from the old beer ads) in 1990. After six months, she'd eaten (yes, eaten--there were only pieces left) two telephones (the ringing disturbed her), destroyed a recliner (she chewed up the wooden frame and ate the stuffing so that when I sat down in it, it fell apart), and chewed a hole in the carpet 2 feet by 3 feet. Total damages of her first six months with me: $2,300. Vet bills (not including spaying & vaccinations--these were trips to make sure the pieces of the stuff she ate weren't going to hurt her): $600. Cost of the dog: $900. (What was I thinking? Oh, yeah. The dog looked really cool. I was young, and cool was important.)
We went to obedience class. I was quickly told that a bull terrier has such an active temperament that they simply cannot be left alone in a house all night (I was on the graveyard shift at the time). After a lot of intensive work, she became a well-behaved, well-mannered dog; however, she never adjusted to my schedule, even though I came home over my lunch hour (at 4 a.m.) to see her.
I ended up giving her to a young man who lived in the neighborhood and had the time to work with her. He got her involved in doing agility trials, which is a sport typically dominated by border collies (but she was a very, very smart dog). She was much better off with him.
In my experience, the obedience class was far more educational for me than it was for the dog, though. I had no idea how confusing my commands (or rather, my attempts at commands) must have been for her. Dogs can only do what you teach them to do--not necessarily what you want them to do. And they will never, never stop being dogs.
I also learned--the hard and expensive way--to research dog breeds before you buy a pup. Every dog won't fit every lifestyle; it's important to find the right fit for both dog and owner. Otherwise, you've got all kinds of frustration and angst where there should be companionship and comfort.
These days, I have a cat. She's much more suited to my current apartment-dwelling urban lifestyle. When I retire and I'm around the house all day, though, I'd like to get a sheltie and try some agility trials. It really looks like fun.