5 years ago when diesel was less expensive then unleaded was the time to really push the diesel powertrain. Its very hard for people to look past the higher per gallon cost and jump into a TDI.
I looked at them and would greatly prefer a jetta TDI vs a Prius, but with the higher cost per gallon, the Volt is still less expensive to operate per month (assuming 50 mpg on jetta).
I think Bloomule is onto something with the diesel generator. The 2015 Volt is rumored to have a longer range EV mode (varying reports have it at 50 - 80 average vs todays 38 average) but the true dagger in the heart to the Prius is the also reported 50 Mpg when not in EV mode.
Ive said the volts benefits are in staying in the EV range. With the projected conventional mpg improvements the benefits will extend far beyond the EV range. At that point you have a much better looking prius on steroids.