I myself have never owned a mix breed, either has my family up until last year when my sister rescued a little pug, spitz and bulldogge mix named Senna.
She is the oddest looking dog I have seen but is the most gentle, loving, affectionate and intelligent dog we have ever owned. She picks things up so quickly it's crazy, even more so than our Shepherds. And she is FAST. Crazy and silly at times, but that could just be my sister's personality inbedded in her dog.. haha. They are quite the pair.
Here is Senna at her old home, the day my sister met her. It still breaks my heart to this day.
Here she is only a few months later with us. I think you can see the difference a little love and care can do to a dog.