Yeah, my 2000 Frieghtliner XL has these, and they have yet to be needed, but compared to maybe breaking a yoke, they are piece of mind. The normal deal when you drop a shaft is about $250 for a roadside service call, plus $85/hr, plus parts. Not to mention "downtime" while waiting for the mech. to get to you, then go and get the parts several times, since they seem to forget what they need, and then the actual repair time itself. I also carry 2 extra U-joints in my side box, plus the tools to change them myself.
These are actually fairly common on heavy equipment in rock quarrys', so that should tell you something about wether or not they work in an offroad situation. Think 300 ton dump truck.