I have one, in the gorgeous Italian Green colour, and I love it. It's very much a girls car though, and my neighbour laughs at DP driving it!
Mines an 09 plate, and it's been fantastic. I've had it 4 1/2 years, done about 25-30,000 miles, and I've maybe spent £600-700 on repairs in that time. Mainly on replacement tyres, but a bit to the suspension/ bushings. Basically speedbump related stuff! It costs me £40 to fill the tank to full, which lasts me for about 350 city miles (3 weeks of driving to work and general shopping). We often have 2 adults in the front and either 1 adult in the back/ 2 teens.
The annoying bits are the height of it, as it's quite low and there are a lot of SUVs on the road nowadays, you can be at dazzle height for the headlights, and sometimes it struggles to get up to speed- you need to be in a lower gear, which is fine, but my DP often forgets as he's used to driving a turbo van that can come off a roundabout in 4th and go straight up to 60, definitely not the case for the Fiat 500! There also were some reports of hill starts not being fantastic, but I've not seen any issues.
All in all, I've LOVED my car, it's been a fantastic bargain and a right workhorse, and I should be able to sell it for very similar to what I bought it for. I'm only planning that as I'm pregnant and don't fancy wrestling with a 3 door!