It depends on how much rail travel you are planning to do, and within what period.
If you are just intending to travel on the bullet train from Tokyo to Kyoto and back again, the 7-day Japan Rail Pass is not really worthwhile.
But if you are travelling Tokyo-Kyoto-Osaka-Hiroshima-Tokyo within a 7-day period, it’s definitely worthwhile.
www.hyperdia.com is a useful website for checking fares, to see whether it would be more cost-effective to buy individual tickets or get a Rail Pass.
The website also shows train times and journey durations.
Narita Airport to Tokyo on the Narita Express: Yen 2,840
Tokyo to Kyoto by Bullet train: Yen 13,400
Kyoto to Tokyo by Bullet train: Yen 13,400
Tokyo to Natrita Airport on the Narita Express: Yen 2,840
Total: Yen 32,440
A 7-day Japan Rail Pass (which can be used for all above journeys): Yen 29,100