Denny's has always been my go to place for quick and cheap eats. But I never walk through the door expecting to be blown away by the food or the service.
As far as diners go, Denny's has long been the American gold standard for cheap and cheerful. We only have a few here, but they are always busy when I go.
Can't really fault it, as long as you are realistic in your expectations. The menu is huge, and I try to get something different every time. They have creative specials that keep things interesting.
My only suggestion would be to skip the hashbrowns. They are always limp and undercooked, even when I ask for them well done. Unless that's your kind of thing.