There's a castle in France, near Dijon, in which one wall, about eight meters high and five wide, is covered with a detailed map of all the area that belonged to that castle at one point in time. It was so detailed you had to stand close to it, two or three feet, to read it. I have never seen so much information in one map.
What do you mean by custom maps? What do you include/exclude from them, and how do you make them?
I use tools such as Google Maps and Open Street Maps, and I cut/paste, mostly just by eyeball, with the relevant things I might be interested in - the train stations of Rome, for example, and so on. Google Earth is also great for this, but I confess that the last one I made for myself (Berlin), I just screengrabbed and pasted my friends # in place, with the train details, etc.
There's something comforting about knowing how to get around when the power goes out. Since I enjoy roaming adventures requiring navigation, making little maps for myself is basically just how I roll .. got me where I needed to be.
I do enjoy digital life, but you know what else rocks, apropos walking in new cities[0]?
Custom maps, printed.
Its fun to look at the little piles of maps I've made for myself over the years, from Frome to Rome, Berlin to Pristine ..
In fact, I think I prefer printed maps over digital, for personal adventure/navigation needs.
([0] Disclaimer: Got navionics and predict wind running on multiple tablets..)