A common topic on lonelyplanet.com (Thorn Tree discussion board). General concensus is it's not worth it for a short trip, due to cost, and requirements for registration and insurance (e.g., having a local address). Plus the need to sell the car when you're through. For long term rentals, contact the rental companies direclty for special rates vs. just using the Web sites (which may not give you a rate for over 30 days, anyway). Be sure you have insurance. If you use that provided by your creit card make sure you can be covered for that length of time. Your home insurance company may have ideas for insurance in the States, unless you use that sold by the rental company.