Here are some "old timer" Ford diesel owners you can do a search in the archives for on TDS who have had Gear Vendors units installed in their Superduties:
-FastForward - aka Steve Roper
-Fyoung - aka Frederick Young
-crroger (or roger45), forget which, but he has one on his F-550
What might be of interest about the list above is that it is more than 6 years old. I made it back in 2000 or 2001, so if those guys still have the GV, they will have had it for a long enough period of time to provide some long term experienced, feed back. Of this list, I believe that most, if not all, had automatic transmissions as well.
For me, the GV would have to last without breakdown, and without causing another component to breakdown, at least enough miles to justify it's initial cost and the fuel supposedly saved by using it, for it to be worthwhile.
Some people have remorsfully reported a zero gain in fuel economy. Some have glowingly boasted of a 3 mpg gain in fuel economy. Taking an average of what user reports I've read over the last decade, while ignoring the advertising claims of GV, it seems more realistic to expect a 1 mpg gain in economy using double overdrive. But to give GV marketing every benefit of the doubt, let's increase that to 1.5.
At $3.00 a gallon for diesel, it will take me approximately 100,000 operational miles to recover the initial cost for the GV. Therefore, the GV had better last, trouble free, without causing any other costly problems in the transmission or driveline, for at least 100,000 miles.
At that point, other than the maintenance costs for frequent (and somewhat expensive GM Synthetic) lubricant changes, the GV will have finally recovered it's inital cost. It will take a few thousand more miles to recover the maintenance costs.
Dave Whitmer was also on my long ago list, but I see that he is already here, speaking for himself. Back in 2001, I recall he complained of electrical problems. With his obsession for higher mileage gains, if the GV is not something he is using today, that would cause me to think.
Most of the complaints that I have read of over the years are summarized by "The GV burned up"...(overheated). The GV does not get enough lubrication if engaged while coasting downhill. It seems that the folks that have had success with the GV have already known ahead of time to disengage the GV at the crest of a hill, and coast down without the GV engaged.
crrroger (or roger45), is one owner I know of who both tows heavy and has managed to keep his GV alive and well. I'm certain that he even had to accept a downgrade in UJoints on his truck, since Ford equipped his 550 with 1480 joints, and the largest UJoint a GV yoke was capable of holding was a 1350. And he has an automatic. He'd be a good person to talk to about it.