Take the bus from near Pompei Scavi station, the station most people use to visit the Pompeii ruins, remembering to buy a ticket in the newstand in the station first (18 EUR).
The bus takes you along a rather chaotic autostrada, before climbing the mountain. The lower slopes are covered in rather unattractive villas, and rubbish, but as you climb the surroundings become more pretty as do the views. The corners are steep and the bus driver knows the road well, so expect some hair raising driving.
For some reason the bus takes you to an isolated cafe, where you are persuded into getting out and meeting an old man who tells you about the mountain before pitching his book. The book is interesting but overpriced. After that you reach the car park, where you need to buy a 6.5 EUR ticket to reach the top. The walk is short and steep but easy enough under foot. It was cold when we visited and the jacket came in handy. Once at the top, take in the huge views all around, and stare into the crater. There are still fumaroles steaming away at the side of the crater and an occasional whiff of sulpher, so the volcano is only slumbering lightly between active phases, and is by no means dead. Although the biggest and most famous eruption was a long long time ago, there's still the possibility of more. When you see the scale of the development on all sides of the mountain, you'll understand why this is considered such a dangerous volcano, as even a small eruption would destroy large populated areas.
The bus allows you just over an hour before returning, which is enough time to walk the length of the path (you can't leave the path and circle the crater). As long as it's not the last bus of the day, you could stay longer and catch the next bus, although aside from the views and the gift shops, there's not much to do. It is interesting looking at the lava and other volcanic material to see which has been weathered and looks old, and what still looks freshly made from eruptions within living memory.
We visited the mountain after visiting Pompei to see what happens when you are on the receiving end of an angry volcano. At this time Vesivuis was still the world's most famous volcano, and Eyjafjallajokull ash cloud related chaos was still two days away. Little did we just how much volcanos would change the rest of our holiday.