I've spent a lot of time in Germany, and I plan on spending even more.
If you go, there are a few really cool places you can check out. My favorite is the Rhein, and the area around the German/French border, but the Rhein between Koblenz and Frankfurt is the best, most accessible, and most interesting. Loads of castles, but the smaller, independent ones. Beautiful river, beautiful drives, wine, food, pretty much everything. If you are looking for less expensive or easy access, stay in Koblenz.
North of that portion of the Rhein is Cologne (Koln). Decent city, with a decent street life, and one really big cathedral.
South of that along the mountains, is Neuschwanstein. Absolutely recommend that place. It's the place that Disneyland was modeled after.
Berlin is great, too. The history is different there, and it feels obvious, however it is a different city from 15 years ago. You can't really see the wall anymore.
Dresden was a lot of fun, great place.
Hamburg, and the smaller city of Lubeck, are probably the most underrated cities in Germany. With no big sight to draw in tourists, the cities feel a lot less touristy. Lubeck is one of those old towns, and it is really great if you like that sort of thing (I do). If you are more into nightlife and that, the reeperbahn in Hamburg is famous.
Lubeck
Depends what you are into. If you prefer big cities, then Berlin and perhaps Hamburg are for you. If you like mountains and/or the ocean, then go there. If you like beer, well, go everywhere, because each place has their specialties.