Sonic is a good standby when you want fast food but want something a bit different - we rarely eat there, so it's an interesting change. Sure, there are burgers and fries, but they always seem to have something different on the menu as well (plus they serve breakfast all day - something those other guys aren't doing at this point, unless you're stopping at Jack in the Box).
Now if only what they brought out looked like what was on the menu - but that's not unique to Sonic or anything.