Use tools like JRailPass Calculator or Navitime to plug in your itinerary and see the exact yen-for-yen comparison.
If you are mostly staying in Tokyo or Osaka and doing local day trips, the pass is a waste of money. when to buy jr pass
The fastest bullet trains require an extra "supplemental" fee even with the pass; individual tickets let you book these directly. Pro-Tips for Timing Use tools like JRailPass Calculator or Navitime to