produce very little thrust
. The number quoted by NASA is equal to "10 US quarters". That's great for unmanned interstelar missions, but unsuitable for any environment with an atmosphere. These thrusters put-put along, accelerating a mass over a period of weeks or years. Yeah, after a few years you're moving along at a pretty good clip, near lightspeed, but that's not the sort of performance that soccer moms want in their station wagons.
Another practical issue is the high power requirement of ion drives. It's okay for NASA to equip a spacecraft with a little nuclear powerplant. They do it routinely, but I wouldn't expect Toyota to start splitting atoms any time soon. The Japanese are, somewhat understandably, reluctant users of nuclear technology.
The claim of X-number of MPG is probably misleading. From my understandine of ion drive, very little mass is accelerated to very high velocities to achieve thrust. They're probably talking about a miles per gallon of liquid xenon. Yep, you don't use much xenon, but the electricity required for ionization has to come from somewhere. You can have huge solar arrays for a mission close to the sun, or a little "Mr. Nuclear" nuke plant, neither of which are practical for the family car.
By the way, Ion drives are not new. NASA built one back in the Mercury/Redstone days. We're talking almost fifty years. In the world of technology, a single decade is a long time. Several decades don't add up to "new".
Anyway, ion drives aren't going to free the world from fossil fuels, but this is important technology. We need to use energy more wisely than we're using it today, and we need to look at all the available technologies. However, ion engines are suitable for spacecraft, not minivans.