The 393 has been named several different titles by Manfrotto and has been the recipient of at least two number designations (3421 and 393) and these changes have been since July of 2007 when I purchased my unit.
I am extremely impressed with all aspects of this mount for use with long lenses and especially for following moving subjects. I can easily balance my 40D camera with a 400mm f/5.6L lens and shoot from either a sturdy monopod (I use a Manfrotto 681) or from an equally sturdy tripod (I use a Giottos MT-8180). The camera/lens can be balanced so that finger pressure will move it but, it will not move unless pressure is applied.
Although I use it with the 400mm f/5.6L package; I would have no hesitation in opting to use the 393 for a much heavier lens. here is a site on Philippine Bird Photography which illustrates the 393 with some very large and heavy lenses:
http://www.pbase.com/liquidstone/testequipment
Here is a two part review which was posted on the Nikonian Website...
http://www.nikonians.org/html/resources/non-nikon_articles/manfrotto_393/393_1.html
http://www.nikonians.org/html/resources/non-nikon_articles/manfrotto_393/393_2.html
Now, I have a few comments from my personal use of this gimbal rig.
1. The way that Manfrotto illustrates the use of this rig is extremely clumsy for me even with the smaller 400mm f/5.6L lens and would be almost impossible if used with a larger lens such as is illustrated in the equipment section of the Philippine Bird Website. Manfrotto persists in showing the camera/lens mounted so that the inside "U" shaped mount is on the top and the camera is slung under the mount. I could never work with this configuration and I don't know anyone who uses the 393 this way.
Manfrotto illustration:
http://www.adorama.com/BG3421.html?searchinfo=manfrotto 393&item_no=1
2. Manfrotto will tell you that it is impossible to mount a flash using the 393 however, I found an illustration for a flash bracket simply fabricated from an inexpensive piece of flat aluminum stock available at most home improvement centers and a couple of nuts and bolts. It is fabricated by simply bending the aluminum bar into a "U" shape and mounting it inverted over the camera/lens to the "U" shaped inside arm.
I will be happy to send the illustration to anyone who contacts me but, placing an image on this site is more trouble than I wish to go through.
3. I prefer using a good Arca compatible ball head when doing other than following moving subjects.
I really like this unit and the price is right... By the way, it comes with its own mounting plate. Unfortunately the quick release is not Arca compatible. But, then again, nothing distributed by Manfrotto is Arca compatible....
but it very sturdy and cheaper the the Wimberley Gimbal II