I've owned many many dogs in my lifetime, and two of my favorites are mutts. the first was back when my parents were breeding and somehow we ended up with a Rott/Shephard mix. I kept one of the larger males and we gave away the rest. 120+ lbs later he finally lived up to the name Zeus and was the kindest, gentlest dog I've ever had.
My wife's dog is either a boxer/pit or boxer/shep mix (according to vet) who is also one of the biggest babies and often tries to crawl into my lap during a thunderstorm. Neither dog was as cagey or as snippy as my purebred collie. The only thing I have a hard time with is the prices.... I can't see paying hundreds to thousands of dollars for a designer dog. Any mutts I've ever owned were either given to me, or I had them for the cost of the shots.