I went inter-railing the summer after I finished school and to this day it was the best experiencing I have had travelling - I loved it!
Having all of your travel paid for in advance took away a lot of the financial stresses as I knew I didn't have to budget for this.
The inter-rail network is also really extensive, so the choice of places to go is amazing. And there's no pressure, if you're not wild about somewhere you can just hop on a train and go somewhere else. This is my kind of travel.
My only tip is to just relax and enjoy it. It's quite a unique way to get around, and although it may be a little slower than flying, it gives you the opportunity to look out of the window and see some amazing scenery that you don't have the opportunity to do on a plane.
Oh - and definitely check out Eastern Europe... Hungary, Serbia and Croatia in particular are beautiful.