Eurail pass

I travelled with afirst class pass because you can only get a 2nd class one when you are under 26. I was glad to have it. It meant pretty much always getting a seat, more room to move and more comfort etc.

If you are going to be travelling in peak period (july & august) and you can afford it, it would proberbly be worth paying the extra so you don't have to worry so much about full 2nd class trains.

Don't buy a Eurail pass. It ends up being just as much money with less flexibility. This is especially true if you are travelling within one or two countries. You'll spend the same amount buying individual one-way tickets without committing yourself to several train days per month.

Don't bother with first class. Its hard to tell the difference.

The difference between First and Second is not as significant as it is for planes. But the same applies with prices...
First is slightly more confortable with some more leg space. It may be slightly quieter too.