Never thought they were worth having an extra piece of equipment to carry. Don't think they do anything you can't do better with a tripod and a home made Steady cam is much more versatil in my opinion.
I've seen 'TrekPods' at my local camera store. It is about the size of a monopod but works as a tripod with a very long center column. I can see this being used with a lightweight camera for lowlight photography such as fashion shows or fireworks shows. I would still rather use a tripod whenever possible though.
Actually both Blake and Kelly are correct. They are mainly for places where you can not take a tripod. They are not meant as tripod replacements. Watch any NFL game and you will see sport photographers on the sidelines using them. I have one and have had a few occasions where it came in handy.
Boy I sure wish i had one on many occasions. I could almost bet that some of my shots would have come out much sharper if I had one.
I'll shoot the kids soccer, baseball and football games with my tripod folded up and with one leg down. Then if I want the tripod, BAM! it's there!
I have a SunPak VersiPod 2 monopod. It has 3 legs that open up and becomes a tripod. Is it as steady as a regular tripod? No! But, it is helpful when I need something to help keep the camera steady. It also doubles as a walking stick.