Yup, I've got it. It's the only way to bathymetric info with these plotters, so they are pretty much mandatory if you fish. Otherwise you just get the standard noaa nav charts. No bottom structure to speak of.
It's also a nice to have if you have a garmin AP. It lets you click a destination and it will plot a course there for you. I don't always like the course, but it gets you a good estimate of distance.
You can find them pretty cheaply since they are easy to counterfeit, so beware. But there are good deals on legal copies.