I think the popularity of these vehicles will remain low until there are natural gas fueling stations all over.
Who wants to own a vehicle that they can't take on a long trip for fear of not being able to get fuel.
There is getting to be quite a reliable network on the nation’s highway system.
Trips need to be planned, but much of the US would be available for a long trip. Duel-fuel would still fit most peoples needs at this point.
http://www.afdc.energy.gov/fuels/natural_gas_locations.html
LNG makes the most sense for trucks and other high-usage commercial vehicles.
The truck conversion kits enable trucks to be "dual fuel", diesel/LNG. The truck would run on LNG (the engine needs a little bit of diesel mixed in for proper operation) until the LNG tank runs dry, then continue on diesel. If long-haul trucks are made LNG-ready first, then you would just need LNG pumps at truck stops every few hundred miles on major interstates.