For navigation on a road, you shouldn't really need a compass even.
It can't hurt, but it isn't really vital. If you know to read signs, check your tripmeter, and can tell left from right you should really be pretty well okay.
That being said, having a GPS device is not a bad deal... I have one in my cell phone that does turn by turn directions (blackberry) with voiceover, and we got a tomtom one for the car this christmas. The only thing I would be concerned with is how well the device route plans for bicycles, and also how you would keep it charged. They do have a certain demand for battery power, even the Garmin Edge's consumption is such that you couldn't plan for a multi-day tour with one without having to consistently recharge (daily?)