A monopod is not a complete substitute for a tripod, but it can be very useful in some circumstances.
I tend to use a monopod with longer focal length lenses when I need to be mobile or when the subject is moving around, like aircraft or birds in flight. The monopod provides an extra point of support, which can increase stability, even with an IS or VR lens. It also helps with the actual weight, which with a monster like my 200-400mm f/4 Nikkor is quite important...