A route (for turn-by-turn) is stored as a series of points. The unit can decide how to get to those points... Hence the problem. If you are in a remote area with few roads, it'll probably work fine, as there won't be any other options. In town, or on roads that aren't mapped properly all bets are off.

As far as battery life, if you're not using the backlight it'll last all day on a set of Sanyo Eneloop rechargeables. Bring a spare set for the next day or an emergency. You can also buy alkalines at most stores. The backlight is adjustable brightness. Using it on a low brightness at night doesn't drain the battery too badly.

It has a mini-usb port for power and data transfer, but it won't charge the batteries from it. Too bad.