My very first 156 was Nuvola and I loved it. I cannot tell you how many 'looks' it got when I was driving around. It really does attract attention. I would definitely recommend it.
On a new car it's very expensive (about £1300 option) but if you're buying 2nd hand, try and find a Nuvola car - although they are rare.
Repair-wise, Nuvola comes out of the paint tin as 'Nuvola' - by which I mean it doesn't have to be sprayed as a base, then a pearl. It can be sprayed straight away as Nuvola colour, so labour costs for spraying shouldn't be more.
However, from my experience it is much harder to get a right match for Nuvola. Even Alfa dealers have difficulty. The only option often being that rather than spraying a door or a wing for example, quite often, the whole side of the car has to be sprayed, as it is very difficult to match just one panel to the adjacent panel.
Despite this, I would go for Nuvola again. Photos don't do it justice - you really have to see a Nuvola car on the move for the full effect.