In a previous life I worked for an ad agency and we always tried to steer our clients away from billboards. The cost is too high and the ROI is too low. They're usually an ego buy.
Generally I want to agree, but it depends on the product. Some things are custom made for billboards, like "McDonald's, next exit." Anything that's not primarily about location (next exit), is probably a waste, or would require a massive purchase of space to flood a market, which, like you said, would probably result in a lousy ROI.