We visited the stadium whilst on a day trip to Naples as football mad son wanted to go. Really really disappointed with the area around the ground - dirty and run down.

I have visited this ground 6 times over the past 18 years.  Most recently in May 2016 when Napoli secured champions league football for the 2016-17 season and their striker broke the record for the number of goals in a season (36).
It was a joy and I would recommend all to visit and experience something truly unique.

This stadium and football club has history,  great atmosphere  (nothing else like it that I've experienced ), friendly and welcoming fans and the style of football that is played is attractive and attacking.

History:
The stadium is old and in need of repair.  Thus there are few snack and drink outlets in the ground.  The WC's are old and not the best.  The owner of Napoli is trying very hard to secure funding from the local government who own the stadium to upgrade the facilities. Built in 1959 it was all standing and enjoyed a peak attendance of 129000. Then it was renovated for the 1990 world cup.  A roof was installed and seats.  The capacity was then 82000. Since then no upkeep has occurred.  Therefore recently the sections high up had to be fenced off due to safety concerns. The capacity currently is somewhere between 65000 and 70000. Napoli will regularly get over 50000 fans in the ground.

As a team Napoli has suffered from bias from the Italian FA. As a result often the fans are barred from certain games (namely Roma and Juventus ). The Italian FA has policy of protecting the more sesuccessful clubs such as Juventus , Milan and Inter. Napoli doors not have many friends in Serie A. As a result expect jeers and boos during the national anthem. Remember that Neopolitans don't feel as if they are Italian but rather as Neopolitan or from the Kingdom of the 2 Scillys.    They have established 'friendships' (something common in Italy) with Genoa, catania and Palermo. 
They have the highest following both home and away is Serie A.

The Ground:
You will not see any stewards. Seat numbers on the tickets are pointless.  Fans start to turn up 1 to 2 hours before to get the seats they want.
The fans are friendly and you will be safe.  We sat next to 3 women who kindly lent us an umbrella when it started to rain. On the other side there was a family of 4 which included a 4 year old girl.  Watching Napoli is definitely a family affair. The stadium has more women and children then men.  The locals immediately know when you are tourist but you will find that they appreciate that you have selected to watch Napoli. Just don't wear anything that represents another Italian club.
The ground is made up of 5 main sections. 
1st. The away fans section.  This is usually empty or has very few fans in it with the exception of Genoa who are considered as 'friends' of Napoli thus when they play there is a carnival atmosphere.
2nd. Curva A. This is where the hard core fans go to.  Here there will be flares and loud fire works that shake the stadium when set off.  Seats cost between €12 & €18 per match.  However to sit here you really need to know the songs and anthems they sing.
3rd. Curva B. The same as Curva A but the fans here are a touch older.  Still loud with flares and flags.  Once again you need to know the songs to sit here.  Prices are the same as Curva A.
4th. Disitanti.  This is the largest section and goes the length of the pitch facing the dug outs. This entire section is very friendly.  The atmosphere is very good with flags and songs but you are not expected to know them.  Prices for seats are roughly €25.
5th. Posillipo.  This is the section behind the dug outs where the owner and major sponsors sit.  The most expensive seats. It has the media section behind,  visitors to one side and Curva A to the other. Once again they will sing in this area.

Getting there:
The city's metro system serves the astra very well.  After the game there are plenty of trains waiting to aid you away.  Tickets cost €1.5 and cover 90 minutes of transport on all of the city's public transport.

I watched Napoli-Juventus game in end of September, it was a good game!
but the stadium is not up to standards. It needs a massive renovation. Seats are too high and uncomfortable, and very dusty!! my pants turned white after the match!
no facilities provided, only small canteen for drinks & snacks.
ticket is cheap, just 25Euro.
easy to access by metro.