Is the Eurail Pass worth it?

My sister and I are going backpacking this summer. We are spending a month touring London, France, Italy, Greece, and Croatia. I was looking into Eurail passes and I found this:
http://www.studentpasses.com/eurail-eurailselect.htm
I’d use it to cover France, Italy, and Greece – are they adjoining?
As far as I can tell, it means we can spend any 10 days within a two month period train-traveling, for about $429 (we’re both student aged).
Am I right? Is that what it means?
Also, given that we’ll be spending about 2 days + travel time in each of the 6–8 cities we want to visit, is this the right pass, and does this seem worth it for the money?
I think TGV tickets would be an additional $11 per trip, but they cost about $100 without a pass.
I’m so confused about all of this. Please, someone enlighten me!