I am a British pilot, living in Holland; and, pretty much exactly half of my flying hours have been in the USA. It's a wonderful place for aviation, and my FAA license is invaluable, so getting one will not be a bad thing at all!
The knowledge test will be a piece of cake for you if you have already passed the JAA exams; just get a book like Rod Machado's PPL handbook, and the Gleim questions book. If you can consistently get high scores on those, you'll be able to walk through with no trouble once you get to the USA.
Valid points are raised about using the license in Europe; try looking into the license conversion route and finding out what would be required to convert your FAA into a JAA on your return; then, you'll have both!
After you've flown in the USA, you'll wonder why you didn't do it sooner
