We took ours in the hand baggage - but we made sure it was well charged up in case we were asked to switch it on (we weren't). The battery life of sat nav's tends to be much shorter than other devices that are designed to be used on battery power.
Last time we went we put in the hold luggage and I - stupidly - didn't take much notice of where I put it, leading to every bag being opened in the car park.
Also - and I don't know if this is a problem with other sat navs (ours is a Garmin) but it takes ages to wake up and realise we are in Florida when we first put it on. Last time it was about 15 minutes. Next time I will be switching it on at te earliest opportunity - probably as we queue for the car.