I don't remember it being a "sleeper"; I recall it stirring quite a bit of controversy at the time. It came out in the pre-Columbine "Going Postal" era. But agreed: it's worth a watch.
I was very surprised to learn that Joel Schumacher directed Falling Down.
It’s a smaller movie, given the talent involved, but very interesting. The controversy undermined the viability of its popular legacy.
Kind of like Passengers (half kidding, but the controversy on this one always felt like an inadvertent bend of timeliness).