Hi Lester.I can't tell you much about the JR railpass because every time we have priced it for our particular type of travel it has been, at best, the same price as buying all the trips separately.
Our experience has been that unless you are traveling great distances in a short period of time, it's not worth it. For instance, we travel from Tokyo to Osaka on day one, and from Osaka back to Tokyo on day 9, so we're outside the 1 week pass period, and don't get any benefit from it.
We also find that it is usually not the JR lines we are wanting to use within Tokyo, so we don't get any benefit there.
My suggestion is to look at the pricing, listed here on Japan-Guide, and also use Hyperdia.com to figure out what the trips will cost you without a pass, and calculate whether it's worth it for your particular situation. If it is close, I would opt against it.
Having said all that, the train system is great, even during the holidays when things tend to be LESS busy than usual rather than more busy.
This time of year, I would suggest booking your reserved seats on any long train trips as soon as you touch down in Japan, just to make sure you aren't disappointed!
Last year, we accidentally stumbled onto one of the busy holiday days when all the locals were going to visit their families, and it was standing room only for 3 hours all the way to Tokyo from Osaka... but that's only the few days leading up to New Years.
My understanding is that this year it is 29-31 December when the trains are busy busy busy! We're traveling on the first of January this year, so we should be fine.
I also understand there's a special 3 day ticket which can be pretty much used on any trains during the new years time, and it is cheap, but I can't remember the name of it. Maybe someone out there will know, however.