I regularly use two monopods (not at the same time), a Manfrotto 679 and an ART134. The ART134 is brilliant, and I use it either with the RC2 quick release head, or more commonly the 234 tilt head. In some situations, just having the closed 134 hanging from the bottom of the camera as ballast provides enough stabilisation coupled with manoeuverability. There are times when only a tripod will do, but it's unreasonable to disparage a monopod for having only one leg.