I haven't tried it yet, but my sister and mom both have Pops. Sis got it first. Really wanted a Cooper, but it was too expensive. Dealer next to Fiat had a used one so we checked both out. The little Fiat actually has a lot more usable space. Because you sit up taller, the leg room is a bit more usable. Granted, with both cars, if you're 6'+, only little kids can fit in the back. But, if you're roughly 5'9" and under, you can fit a similar height individual in the back comfortably. Plus, there's quite a bit more trunk space than in the Cooper.
Yeah, the regular Fiat's are slow, but still pretty fun. I'm anxious to try an Abarth cuz I can see how it's a real fun little thing.