I have a Garmin eTrex Legend and did buy the Garmin Topo CDs to download selected maps for areas I plan to hunt or travel in. It does provide a amazing detail of the topography to give you details that doesn't come with the base map.
The CDs were just under $100.
I do have to download over other maps when I go to other areas, just because there's a limit of how much memory can hold. But I've put maps for maybe 1/2 of the state of MN on my GPS on at once. You still have roads and large features in other areas of the country that you haven't downloaded detail maps for. I think it's a wise investment. If it can help you find your way safely and not have to spend cold night in the woods alone how much is that worth to you? It's worth much more than the extra $100 bucks to me I guarantee. Of course, as said earlier bring a backup compass and topo paper map in case something happens to break the electronics. We all know nothing electronic is unbreakable.
Be safe.