I've seen the free Navionics app in action on LOZ and wasn't impressed, but like anything, you get what you pay for. They're using pre-impoundment cartography, so the chances of those contour lines accurately depicting the bottom aren't going to be good in many places. But for the low cost that's good enough for some. Garmin's lakevu hd ultra maps give you 6" contour lines generated by their survey boats within the last few years using scanning sonar, and the ability to color shade different depth ranges is pretty killer... you just gotta pay to play.