For 6 months in 2000 I lived in Rome, and travelled on many a nation's train system. I'd have to say that Italy's (FS, Ferrovie dello Stato) varied the most in quality. The newest Eurostar Italia (the 600 series?) might be the nicest train in all Europe, but the quality varied after that.
The Intercity trains were usually not bad, but generally most Italian trains seemed at least 20 years old. They are often late, and the cheaper the train, the more stops it makes.
However, Mussolini did make the trains run on time, and they are fairly reliable. If you are travelling around a holiday, they can be quite crowded howevr. I stood outside the bathroom for 3 hrs on good friday....
Just be on your guard in some of the larger stations (Roma Termini, Napoli) for undesirables, Naples can be quite interesting after dark in the 'hood around the station.
Also, bring your own food -- the restaurant cars are very pricey and very mediocre, compared to the rest of Italy.