I have a satnav and use it in Ireland on and off, mostly when I need to find a particular address.
I travel to the US each year as well and have been thinking of getting a cheap one there as well (you could get refurbished ones from Radioshack for $99), but I wouldn't know why you would actually need one, unless of course you have a lot of addresses in busy cities to visit.
The road signage in the US is second to none and I have never been lost. If you know where you are going in advance , print out a map from google and you will not go wrong.