There are many pros/cons to the hybrid argument - but bottom-line, the technology is in its infancy. The big issue is if the hybrid can serve as a bridge to an electric/fuel cell vehicle.
Battery life is a significant issue with hybrids. At the moment, Toyota isn’t supportive of the Prus plug-in conversions as the standard hybrid’s programming is designed to be easy on the battery… that is, no deep charge/discharge cycles. A plug-in will necessarily be able to do that to be effective. If you continually deep-cycle the batteries with a daily commute, I’m sure you’d see a much shorter battery life than the crrent hybrids. And noone hs commented on how battery degradation has affected the mileage of older hybrids, either. I think this is a major reason for concern when thinking of buying an older hybrid… I mean, I’d bet many people on this board would have no problem buying a mid-90’s Honda as a beater, but I think these same people would think twice about buying a 2001 Prius in 2015 if they knew that a $5k battery pack replacement was needed.
In all, I think we’ll get nowhere in terms of a real reduction in energy costs/carbon reduction unless we have a COMPREHENSIVE energy policy… stop-gaps like a plug-in car only shifts the problem elsewhere.