Just got home from Italy last night, and am a big AMEX user. I would say that probably 65% of merchants accept AMEX, but there are many that will only take Visa/MC.

Also, FWIW, every time I tried to use my AMEX at the Trenitalia kiosk, the transaction would fail so I ended up using my MC on all of my train ticket purchases. I bought my tickets from the self service kiosks, so you may have better luck with AMEX at the manned ticket counters (which sometimes have a considerable line).

Safe travels!

Bryan