Hi, Keyne:
Yes, I have, and it was fun, every time.
The problems are pretty clear regarding 6x6 trucks: the lenght and the drag. Obviously, you can have a very long truck, 4x2, 4x4, 6x2, etc., and the turning problem is the same.
I was able to drive without trouble through the downtown streets of my city with the 12 m long 6x6 Tatra, though at least a couple of fence posts was blown up by the buggy hung in the back when trying to get to the entrance of the piece of ground in the country.
The drag (and all the related costs) of the tandem is inherent to the configuration so, if you want the extra traction, you have to pay for extra tires, extra parts, extra oil, etc.
The Merkabah has a fantastic steering, it can turn pretty tight despite of its driven front axle. The Atego was very capable too. I do not remember well about the Zetros, it was fun, though. For every truck the situation is different, of course, and I think the MAN trucks, especially the older ones, have Mercedes front axles (or at least the same design) and they steer similarly.
If you have a crush for the Action Mobile MAN and you have the money, go for it. If you don't have the money, then build one yourself. Anyone can do it, I am proof of it.
Cheers.
Pairoa