People can arrive without any problem only till the punta del Filosofo (2980m).
Hikers that want to get the summit or go out from this delimited area MUST be leaded by a guide ! (You can avoid this limit asking a special permission to the Catania department of Club Alpino Italiano )
Mountain shoes needed, the hike has a cost of 55€ (including telpher, taxi and guide.
From Punta del Filosofo to the summit you have to walk for about 1hour and a half (difference in elevation ~ 400m)
I attempted to hike to the summit crater on 5 June 2015 from the Nicolosi side. The gondola is running and from the upper station it's possible to either hike up or take a bus to a certain point. The trail to the summit crater is roped off. I was informed by the Italian guides that it was prohibited to everyone (including guides), but then saw a guided group head up the summit trail...take that for what it's worth.