I find monopods best for square format or rotating back cameras.
I have a Manfrotto monopod with an accessory quick shoe release. It is surprisingly finicky to mount a camera on .
The models that have built in, mini-tripod legs can be handy when there is no wind. They are a decent substitute for something like a camera stand, but they certainly are no substitute for a tripod.
They also can be used as a light stand for a portable flash.
They are relatively expensive.