My recent trip to Connecticut also marked the debut of the use of my Eee, Incubus, as a GPS device. I used the copy of Microsoft Streets & Trips and the USB GPS that came with it, just as I would on my laptop.
My conclusion: the Eee is virtually the perfect device for this. That it's small and rugged enough to not worry that every time I bump it the screen is going to get messed up makes it ideal for use by the passenger in the car. The long battery life is good, though not quite good enough for a full day's puttering around, so I did still need to plug it in in the car some of the time. The screen is a good size for seeing map and directions, and having the full GPS software, with mouse and keyboard, makes it a lot easier to do things like add a stop on the fly (as in, "we need to stop for gas on the way there, what's near our path?").
I did have to use the Full Screen mode which is a little limited in Streets & Trips, being as it's more geared for the situation where you're alone in the car. But I could always hop back out of it as needed. Sometimes S&T is a tiny bit clunky because of it being geared towards a full desktop, but having the full features of that desktop interface when you need them is very good.