When In-n-Out first opened up in the Phoenix area, I think almost 20 years ago, there were only a handful of locations, and they were always busy, with cars wrapped around the block. In-n-Out wasn't new to me as I have had them in Cali, so I waited until the hype died down, but tried them. Like you say, they were good, just nothing outstanding. I still go every now and again. They are everywhere now.
The same exact thing happened with 5 Guys, Fat Burger, Smash Burger, Culver's and most recently, White Castle, when they all came to Phoenix. People freaked out for a while and raved ab