Is jr rail pass worth it?

We are staying in tokyo for 8 days and want to explore tokyo as well as doing day trips to Kawagoe, nikko and mt Fuji (a coach tour?). Should we buy a jr rail pass?

If you are talking about the nationwide Japan Rail pass (such as 7-day or 14-day), definitely no. The nationwide 7-day Japan Rail Pass makes sense only if you are doing at least Tokyo - Kyoto/Osaka return trip with in a 7-day period.

Kawagoe is a commuting distance.

For Nikko, you can buy non-JR ticket or pass (Tobu Railway):
http://www.japan-guide.com/e/e3805.html

If you do Mt. Fuji by a tour you don't need any rail ticket, and if you want to travel on your own to Lake Kawaguchi area or to Hakone, you can simply buy rail tickets (or Hakone Free Pass by Odakyu Railwayif you go to Hakone).

Thanks for the advice. Should we buy the 3 day Kanto rail pass.?

If you travel to Chuzenji area in Nikko and you choose Hakone to see Mt. Fuji,
a combination of
a Marugoto Nikko Tobu Free Pass / a Tobu All Nikko Pass
+ Tobu Ltd. Express tickets in a Nikko roundtrip
+ regular tickets for a Kawagoe roundtrip
+ a Hakone Freepass used from an Odakyu Station in Tokyo
sounds better than
a combination of
a JR Kanto Area Pass used
on JR - Tobu through trains in a Nikko roundtrip
and on JR trains for a Kawagoe roundtrip
+ a Hakone Freepass used from Odawara Station.

If you travel only areas near Tobu-nikko station in Nikko
or if you choose Kawaguchiko Area to see Mt. Fuji,
a JR Kanto Area Pass may be a good choice.

Please note that a validity term of a JR Kanto Area Pass should be 3 consecutive days; for example, you cannot use a JR Kanto Area Pass on both Day 4 and Day 7.