I use a Manfrotto 681B and it's nice & sturdy.
No problems in the last 3 years.
I use a Manfrotto 681 (which is the 681B in aluminum rather than black color).
This monopod is excellent in almost every way. It is sturdy, tall and really light for a pod with those characteristics. In fact I was researching monopods for a recent trip to Alaska and was surprised that the Manfrotto 681/681B weighed less than many, much more expensive, carbon fiber pods. It is 161 cm tall and weighs just .77 kg. It has a suggested capacity of close to 12 kg.
I can use a Canon 40D, 400mm f/5.6L lens on a Manfrotto 3241 gimbal mount with no problems regarding stability. This is a total package of close to 4 kg. I would have no hesitations in putting a far heavier package on this pod.
There is one drawback to the 681/681B. They are 3 section pods and do not collapse into as compact a package as would a 4 section pod. However, the light weight and the stability of this pod is owed in a large part its to being 3 section rather than 4 section. If you can live with a pod that is about 67 cm long when folded, the Manfrotto 681/681B is a great choice.
BTW, I also use a Kirk Enterprises MPA-1 swivel head on my pod. This is a Manfrotto head that has been modified to take a Kirk Arca Compatible quick release. That way, I can use my Arca Compatible plates and L-bracket from my tripod head.
http://www.kirkphoto.com/tripods.html#monopods
Really Right Stuff also distributes monopod swivel heads with Arca Compatibility.