It is possible to get by with 'just' OSM maps on the GPS - you don't have to buy maps. OSM usefulness varies a lot depending on where you want to go:
rural UK - pretty good, and in some ways better than Garmin maps
suburban UK - not as good as Garmin maps
France - patchy, some areas excellent but many areas have not really been 'explored' by the mappers yet
India - in remote places OSM can be better than any other available map, including paper ones and even including Google Maps.
and so on
Getting them into the GPS is a learning curve, but once you have found a workflow that works, then easy enough.
But if you 'love maps' (by which I assume you mean OS maps) then all GPS vector-based mapping is a bit of a cold shock at first (though I've learned to love it and prefer it to OS in many ways.
Look at the screenshots on
this page to get an idea of what to expect - these show overlaid contours which is fairly advanced stuff.