I've travelled quite extensively to, from, and throughout Italy (although not all the way south). Some of the more local trains (regionali and locali) can be a bit "rustic", but as long as there's no strike, they pretty much run on time, and are quite reasonable (if you're not going far on some days, consider just buying a ticket - it can be cheaper than a supplement, and would save you having to burn through a railpass day). I have taken the old ETR trains that ran as "Eurostar Italia" many times - gorgeous and comfortable - and last time I was there I took a "surprise" Frecciarossa from Roma to Napoli (it hadn't officially opened yet, they were doing a "soft opening" where it wasn't even on the timetable yet), which was also beautifully designed and comfy, although much of it was built on an elevated viaduct, which kind of reduced the charm of seeing the scenery at ground level. The FS is the one thing Mussolini managed to get right!