I've driven one. Zipcar UK (RIP!) had a few Fiat 500 Hybrids and I ended up with one once when every other nearby Zipcar was booked and I had a last minute need for a car.

Given they are a relatively gutless car to begin with (1 litre 3 cylinder 70hp tinpot engine) I did wonder what the zigzag/lightning icon was on the dash so I googled it.

Turns out the system uses a 11Ah lithium battery that lives under the driver/passenger seat that charges through regenerative braking. It gives a small boost during acceleration (mostly at low speeds so it's more for stop-start urban driving), I think it's not much more than a glorified belt around the crankshaft giving a few extra hp.

No appreciable benefit to it that I could feel, but if it's helping us burn fewer dinosaurs then that's all good. (It's still a car but much better than a massive wankpanzer.)