[MetroMPG]

The 2-cyl engine in the Fiat 500 has gotten decent reviews in the couple of English language reports I've read:

...the more constant flow of air keeps the small turbocharger spinning, pushing power up to 84bhp, and just under the magic 100bhp-per-litre mark. Not bad for such a small, yet refined engine.

What’s it like to drive?

Just like a normal 500. [...] Around town, if you keep below 3000rpm, the engine doesn’t feel greatly different to the 1.4. But once you breach that rev threshold, you get an interesting motorbike-esque thrum from under the bonnet. The noise can be a bit tiring on the motorway, but it keeps up with traffic. On a more interesting road, it’s actually pretty good fun, thanks to the 107lb ft of torque, as long as you keep the engine buzzing away under the bonnet.

From: Fiat 500 TwinAir two-cylinder turbo review | evo

On the road, the 500 TwinAir is deeply impressive. Only at idle and under full acceleration are you truly aware of the engine's twin-cylinder format, and even then the noise is gruffly enjoyable and the balancer shaft keeps things four-cylinder smooth. We were able to cruise quietly at 90mph, more conscious of wind than engine noise, though other testers complained of a 'drone' which suggests some variability among early-build cars.

[...]

In summary, the TwinAir shows that amazing advances are possible in the practical efficiency of small cars, and it makes an invaluable addition to Fiat's small-car range — already the lowest-polluting in Europe — that other manufacturers must surely envy. For potential customers it’s a great ownership proposition, and a lot of simple fun.

From: Fiat 500 TwinAir - Road Test First Drive - Autocar.co.uk

We'll never see it in North America.