I have been using the Manfrotto 3421 Long Lens Monopod Bracket for about six months now, both on my Bogen-Manfrotto 681 monopod (which is a really heavy duty pod) and on my Giottos MT-8180 tripod (sturdy and tall carbon fiber model). IMO it is a far better set-up than using a ball head on a monopod and it works equally as well on a sturdy tripod. I use it with a 400mm f/5.6L lens and occasionally with a 300mm f/4L IS lens. I have tested it with my 70-200mm f/4L IS lens and it works just fine. However, it is really overkill for this light lens that has IS assisting you in holding steady. I never use a monopod with shorter lenses. I have a lighter weight but, still sturdy, tripod for that use.
3421 Bogen - Manfrotto Long Lens Monopod Bracket
The Manfrotto illustration shows the 3421 being used with the open side of the "U" shaped support down and the camera mounted beneath the U bracket. However, I use my 3421 with the open side of the inside "U" shaped support up and the camera mounted right-side up. I don't know why Bogen-Manfrotto insists that using the camera under the U bracket is the way to go. For me, it is far easier to have the camera swinging on top of the "U" bracket as in the illustration in this web site under the section "Key Features".
Manfrotto 393 Gimbal head* (1) - Nikonians Product Review
This mount is excellent. I can adjust the balance so that the camera/lens will remain in place unless I move it and then, it moves smoothly with only finger pressure. Using the 3421 and my 400mm f/5.6L lens; I have been able to follow 150+ MPH hydroplane racing boats that were crossing in front of me rather close to my camera position. The 3421 works beautifully, is very well made and once was a great buy!
I don't know what has happened to the price of this gimbal head. When I purchased it, new, in the Summer of 2007; the price (including one camera plate) was right around $100 USD making it far-far less expensive than any of the other competing gimbal type heads such as the Jobu. An 82% price increase in a matter of six months is obscene and I wonder if the falling U.S. dollar accounted for this great raise in price.
When I am shooting with the 3421 and my 400mm lens using a tripod, I attach my 70-200mm f/4L IS lens (on another body) to the underside of the tripod in a holster case (using a carbiner). I attach it to the ring on the underside of the tripod apex. This ring is designed to either attach a weight or to tie down the tripod. However it is a great place to keep my extra camera with the 70-200mm f/4L IS lens safe (from sticky hands of strangers) and secure (from being tripped over as I am shooting with the tripod mounted camera).
I have not as yet figured out a way to jury-rig a flash when I am using the 3421. Obviously, if I mounted the flash on the hotshoe, the 3421 would obstruct the flash when the camera was in the vertical position. I think that I can fabricate a flash bracket which will attach to the lens mounting plate like the Kirk flash brackets attach. Anyway, that is my next project. I like to use flash when shooting wildlife.
I have noticed a Canadian eBay seller who is marketing copies of the Jobu Black Widow and Sidekick gimbal heads. In fact the company's name is "Jobu Design Photographic Equipment". I don't know anything about the quality of these (I suspect) Chinese copies. I am just passing this on for general information.
eBay Store - Jobu Design Photographic Equipment: Gimbals, Bubble Levels, Flash Brackets