I have an old Bogen aluminum two-section style that I bought in 1984. It looks kind of outdated to people today, I suppose, since it's not black. It's pretty old-school looking too, being larger in diameter than modern multi-section models. I don't use it very often these days except with a long lens at longer events, where I know I might end up being there for two to three hours. I've tried it with a same-vintage Cullman ballhead, and while that was "okay", I found that NO ballhead was easier to handle, since every lens I use the monopod with (400,300,200) has its own,solid, rotating tripod foot, and the camera spins between tall and wide format on the tripod collar.
I think if I owned a smaller, more-compact, lightweight model that I "might" be tempted to use it in low-light situations where I might be able to squeeze out another two shutter speeds worth of stability--but then by the same token, once you're down into the slow speed ranges, subject motion blur becomes a huge factor much of the time, so there might not be a real, justifiable net gain.
The thing I think the monopod does best is to keep the weight off your arms and elbows, especially if you are not in good shooting shape, or if you are sitting down at a shooting position and just want to keep the camera/lens combo up and pointed downrange, at events like say baseball games shooting from the stands above 3rd base, or at windsurfing along the Columbia, sitting on the rip-rap and waiting for surfers to pick a line that brings them toward you; if they can see you and the big lens, they will often (subconsciously?) head right toward your camera position.