I had an amazing time. I went to Germany, Austria, Croatia, Italy, back through Munich, Prague, Munich again, (my buddy picked up a hot blonde german girl in Munich) Black forest, France, then up to Nurbergring, then on to Amsterdam.
Best 17 days I have ever spent. I brought about 1500 Euro, and paid for hotels and gas with credit card, the credit cards are hit and miss at some places because US cards do not have the "Chip and Pin" and are signature based instead. Check with your banks for fees before you go. Capital One and Discover both have no foreign transaction fees cards.
ATM machines are pretty easy as well if you don't want to carry a lot of cash with you, I used them in Prague(kroner) and Croatia(kuna) and they worked great.
My favorite places were Bavaria (Munich and Prague) The beer is amazing, I prefer ales over lagers but the lagers in Bavaria are exceptional, the best in the world. Hiking in the black forest was really nice and it was fun to get away from the major cities and tourist areas, One of the best parts of the ED is the ability to go to the smaller off track regions and explore.
Don't try and plan your whole trip out before you get there. You will end up wanting to stay in one city longer and skip another, having some flexibility in your plans goes a long way.
Restaurants take a long time. Usually an hour+ for lunch, 2 for dinner.
Amsterdam is a must as well. Great Museums, and the red light district has to be seen to believed.
Enjoy the Autobahn and avoid the Staus(traffic jams)!