Not very... a monopod can be compared to image stabilization. It will usually do an extremely good job of keeping your camera from moving vertically, but it will still move horizontally quite easily. Even bracing it against a fence, I had difficulty getting very sharp longer exposures at WDW.
Don't get me wrong, it definitely helps, and it's almost mandatory when I drag out my very big and heavy old 400mm lens, but a monopod will not be an effective substitute for a tripod if you want maximum sharpness on very long exposures (like, longer than a second or so.)