I can't tell you whether the pass is worthwhile. That depends on exactly where you'll be going. But there are some resources from The Man in Seat 61, who maintains a comprehensive guide to all things related to trains, to help figure it out:
In particular, the second link goes into a great deal of detail on how to decide whether a pass or individual tickets is the right choice for you.
One issue is that some rail prices in Europe are priced like airfares, with cheap tickets if you book in advance and expensive tickets for last-minute travel. This means individual tickets can be a better value if you book in advance, but the pass may be more worthwhile for a trip where you want maximum flexibility to go with the flow.
A key question is how often you'll be traveling by train. A global continuous pass provides the most flexibility, but is the most expensive. As Seat 61 says, "to make them worthwhile you need to be on a train every day or at least every day or two." If you're likely to stay in one place for longer periods of time, one of the flexi passes would allow you to take advantage of the pass savings for more expensive trips without feeling like you're wasting your pass if you spend some time in a single city. $2,700 buys a lot of transport—about $30/day over a three-month trip—, so that's really only going to be worthwhile if you are traveling by rail quite frequently.
As the articles discuss, a combination of passes may be to your advantage as well, since passes limited to one or a couple countries are cheaper than the universal passes.
Note that rail is not always the best option to get everywhere in Europe, and bus and/or air travel sometimes makes more sense. Dubrovnik, for instance, is not connected to the rail network, should you want to visit it, various Greek islands are reached by ferry or air, and flying around Europe is sometimes less expensive. So I would be open to considering other forms of transportation as needed.