I had an Abarth as my DD for about two years. Traded it in on the BRZ. I rather enjoyed the little car. It had a TON of character. Its engine note was fantastic. It was comfortable with four people once the four people got into the car. It had decent cargo room when you folded the back seats down. It was packaged very, very, well for what it is - a FWD city car.
I rather enjoyed taking it into the city (Dallas) and darting into tiny parking spots. However, it was not RWD, and it had a lot of body roll. I no longer needed a commuter car, because I am now able to work out of my home. So I got the BRZ.
I had about 45,000 miles when I traded in the Abarth. I never had any major issues with the car - just regular maintenance: regular oil changes and one tire change. It didn't hurt that when I went in for service I got to admire the Alfa Romeo 4Cs that they had in the showroom
The rear axle in the Abarth is not the most sophisticated unit, and you will also feel crazy body roll if you are used to the BRZ. My first aggressive corner (coming from an RX8 in the past), felt like I was going to roll the car! The car has fairly good handling limits, it just will feel disconcerting at first.
If babying it and out of Sport Mode - I got about 40mpg on the highway. The gas mileage is very good on this car. The shifter is awful and was the low point in the driving experience. If I was going to keep the car any longer, I would have gotten a short shift kit, and made it less awful.
If I could have both the BRZ and the Abarth, I probably would. Sometimes I miss the big sliding glass roof (didn't have the convertible). You can get a fantastic deal on Abarths, they are a performance bargain for the money. And you can say that you have now owned an "Italian" car!
No experience with Porsches. One of the guys I used to work for had a Cayman, he rather enjoyed it. Never rode in it, but interestingly enough he admired my Abarth.