Although I've used the for ages, I don't find the maps on a GPS very satisfying. There just isn't enough area to give proper context. I consider the GPS map a convenience for cross-checking where I plotted the reading on a paper map (which I never go anywhere without). The GPS map is also handy for casually tracking progress if you're on trail. It really comes up short for serious route finding if you don't have a downloaded track.
Regarding the original question, I have Garmin's 100k maps, which are generally OK for what I described above as well as the National Geographic Topo! 24k maps (now discontinued in that form) and a few other downloadable maps. The 24k maps are OK if your close enough to something that you can zoom all the way in. Otherwise, the greater detail turns to distracting mush.