First thing, Hamburgers are my favorite food. Second, chains are NOT, and I repeat not, the best burgers you can find(in all honesty, this is a food that's better at home unless it's your favorite as then you tend to undercook it due to impatience).
With that said, the best chain hamburgers I've ever found are Backyard Burgers. They even beat out "sit down" restaurants in quality and taste. The Mushroom Swiss is sublime, and I had never found any good ones before it. The meat is big amounts and juicy, not greasy.
Yet, every time I drive by or into one, I'm about the only person there. Why is this? Maybe, just maybe, it's due to their prices being a little higher than the norm, but the burgers are of the quality that makes them more than worth that little extra. You can't go wrong with any of their burgers as it's all high quality. Maybe it's their limited dessert menu, I don't know. I, for one, go get a burger for the burger, not the "hot apple pie" or whatever.
If you've never tried it, give it a shot. If the experience is bad, it's probably due to their worker's incompetence as is the norm with most chains.