I've been here numerous times, and it's usually a rather uneventful affair. I've found the service to be acceptable in regards to timeliness and employee demeanor. Although they offer a rather wide variety of sandwiches, hot dogs, and miscellaneous add-ons, to me Sonic will always be about the tater tots and exotic drinks or slushies.
I go here several times a year, although sometimes dining in your car can get a bit awkward. There is a small outdoor area where you can eat, but Vegas weather doesn't always make that an inviting option.