We bought a 2005 F250 with the V-10 last summer when gas prices were sky high (there was a smart move). One of the things the dealer offered was a natural gas conversion. We bit since in Utah the price of CNG is less than $1/gallon and the conversion cost was relatively inexpensive. We have the Natruell conversion. It's actually manufactured in the Netherlands by Prins and imported and distributed by Natruell. It's my understanding the Natruell is essentially out of business or on hold at the moment.
The kit is designed to integrate with the OBDII system, not fake it out like cheaper kits do.
Pluses -Nothing like filling your tank for less than a $10 bill. My truck is greener and cleaner than your little Prius and tows 12,000 lbs. For the most part the truck runs well, the engine should last longer. Still have the 30 gallon gas tank underneath. When gas was $4/gallon, it was cheaper to drive the truck than it was to drive my car. Haven't noticed a loss of power. There is about a 1 mpg reduction in mileage, from around 9 mpg to around 8 mpg.
Minuses - the check engine light came on. We were told the cats were shot. Took it to the Ford dealer thinking it was under the emissions warranty. They said since we had an aftermarket conversion, the emission warranty was void. The installer says it shouldn't be. Still arguing this one with the installer and Ford. Every once in awhile when taking off from a stop, there's a hesitation before things get going. This only happens when it's running on CNG. We're hoping a change in spark plugs will take care of that. The 10 gallon tank occupies a good portion of the bed and only gets us about 80-90 miles before having to fill up again. We want a 20 gallon tank. It's not an EPA certified kit. If your truck never gets smogged, then no big deal. Ours does. It'll be interesting to see how Utah deals with it in a year. There are changes afoot.
If anyone wants some technical details, let me know and I'll answer as best I can.

Jim
2005 F250
V-10