Good Enough for a Plug-In?

I've been thinking a lot lately about what is good enough for a plug-in hybrid electric car. With Ford deciding to opt out of the Escape Hybrid (they are instead moving the hybrid version into a new model, although it will be similar from what I've seen), I was wondering why no one else has tried to truly duplicate the Prius model.

 Obviously, the Prius is the top selling hybrid car on the road and has been for over a decade. And Toyota has done very well by it. It beats out all other hybrids, with one out of every two hybrids sold in the US being a Prius. That despite the multitude of hybrid models on the road today.

 The truth is, the Prius is still the best of all options. Room for four, best fuel economy, not a luxury price tag, etc... Nothing else is similar. Nothing else beats it out in those three categories.

 So, now with car companies beginning to compete with in the plug-in hybrid category, what's going to be good enough, and what's going to be the best? Is the Volt the best car companies can do? Will the Prius plug-in continue the Toyota domination?

When looking over the specs for the V60 Plug-in Hybrid, where does Volvo try to differentiate itself? They are already warning in the media releases of the cost. The electric only mode is 50 km (31 miles). There is room for four. But does Volvo think that's good enough to compete realistically?