Nice & easy concept but it's like having an ICE & saying don't fill up your gas tank just exchange it with a full gas tank (tethered at that).
I'd like to know how this effects parking, visibility, towing, & snowy conditions while going up hill. I like the idea of exchangeable power supplies but can't see this being viable in it's current configuration. Something that's incorporated in the body of the vehicle would be practical. Guess you have to start somewhere feasible and cheap and run from there. 
Sure, it would be easier to use if it was integrated in the vehicle (like the Chevrolet Volt), but then you would have to carry all the ICE parts all the time. I think the eBuggy is a perfect solution if you travel outside your EV range only a couple of times a year. It would be even better if it has extra space for some luggage that you're likely to carry with you on those occasions.
A similar range-extending trailer was built when the first EVs were sold in the US in the 1990's. See here:
http://evnut.com/rav_longranger.htm To make things easier, the trailer had a steering system which automatically maintained trailer-to-vehicle alignment during backing.
