Do your research before shelling out for a JR Pass. Here's my own rail costs for 4 days in Tokyo (in US dollars for easier understanding):
$35 - Ticket from Narita Airport to Tokyo, includes Suica smartcard with $15 credit
$10 - Additional credit I put on Suica card to travel around Tokyo
$30 - Ticket back to Narita Airport from Tokyo (Using NEX is a rip-off, but it's express and I didn't have time to find cheaper option)
Total cost: About $75 USD.
I went to Nikko for 3 days (2 1/2 hours north of Tokyo):
$30 - Standard train fare to get there
$60 - Shinkansen back to Tokyo - figured I'd spoil myself
Total - $90.
Overall cost: $165 USD. Much cheaper than the $280 7-day Rail Pass. Even if I did buy the pass, I still would not have been able to maximize its usage within 7 days.
So, as people say - it depends where you intend to go with the Rail Pass. Otherwise it's not worth it.
Here's a fare calculator for the Japan rail system, very helpful as long as you know the station names:
ht tp://ww w.hyperdia.com/cgi-english/hyperWeb.cgi
(remove the spaces in the URL - the board won't let me post URLs yet)