A pass works very well in countries that still separate tickets from reservations. In most of central Europe you can still hop on a train for example. And you can get reservations if you want relatively easy, and cheaply.
But France or Italy can be problematic.
My sister and her family were travelling in Europe. 2 Adults, 3 Children. They needed to get from Milano to Spiez, to come and visit me. Because of engineering works Trenitalia had opened the EC trains to Switzerland for booking only two weeks before they were to run. But what was worse, they had neglected to upload the seat inventory in to the European reservation system (Resarail). The result was that, while you could just buy tickets for those trains, you could not get Eurail Reservations. Not even at the ticket office in Milano. They ended up buying just a family ticket to Domodossola (from which you no longer need a reservation) which actually ended up cheaper than 5 Eurail reservations would have cost them.
Then a few days later they continued to Belgium via Germany, and we easily reserved seats for the whole family, for the two ICE trains they needed, for the grand total of 9.90....
So Eurail is good if you are planning to indeed do long stretches in central Europe. I use Interrail (the domestic variant of Eurail) quite often for business trips to Germany (and get a good chuckle from my accountant...) but I would never consider a pass for Italy or France or Spain.