I stayed at the Homewood Suites across from this development over my last trip and was genuinely surprised by the seemingly unending line around the In-n-Out Burger. From just after opening (11A) to about 30-45 minutes before close (1:30 AM), it was fairly consistent each day of the week - minus Christmas of course. One family staying at our place seemed to be super thrilled about having survived the wait one day and came in with close to a dozen bags of food from there, eager to eat it in the dining area of the hotel.
To each their own, I suppose. I'd tried it once at their location in Irving at Las Colinas and was unimpressed.
Their suppliers are definitely on it though. I went out one night maybe around 2 AM for a Whataburger/Shipley's run (don't judge me!), and right behind the restaurant was an 18-wheeler already unloading stuff for the next day.
I should be back in the area in June. I'm curious to see how much the hype may have faded by that point.