True, I've also had Fatburger that was really good and some that were so-so. In LA, they were great, in Atlanta, I never go there because I've never had an even slightly above average burger at the location here.
In my opinion, Five Guys is miles ahead of Sonic. I would never in a million years even consider Sonic if a Five Guys was just around the corner. I only go to Sonic for their fruit smoothies. Those are great.