The Seishun 18 Kippu is a ticket available 3 times a year and it gives you 5 days of unlimited rides on local and rapid JR trains throughout the country. It cost 11500 yens, which is really cheap regarding the generally high price for train transportation in Japan. However, you must be prepared for long rides and crowded trains.
From Tokyo to Kyoto, it took more than 9 hours and we had to change trains many times. We had to always be careful to take the correct train, and sometimes we didn’t have much time for the transfer. Also, most indications were given in Japanese, so we had to be extra vigilant. The website www.hyperdia.com was really handy as I found in advance the times and places for transfers. Japanese trains are -almost- always on time, so with a good planning the trip went smoothly.
Some lines are more crowded than others, so there were a few times we had to run in order to get a seat. On one day (it might be because it was a Saturday) trains were so crowded that we had to stand for almost 3 hours. Not the most comfortable way to travel !
The good point is that we got to see the beautiful scenery, as the train went near Mount Fuji and the sea.