Easy to create a map, the editors are certainly awesome.
Hard to master though... Everyone's first map is awful, and mine's proportions had a complete disregard for movement. I recommend you start out making small maps and don't be afraid to publish your first map; feedback is a doom mapper's dream.
EDIT: If you do decide to get into making maps for Doom, there's 4 main options for map formats.
Doom 2 format is vanilla, has restrictions on map size and it's how ID made their maps.
Boom - Doom 2 format has a little bit more features & removes some vanilla limits.
Hexen format has more features than Doom 2 format but is a weirder format and generally less used.
UDMF has the most features. You can do some crazy stuff if you use UDMF to it's fullest.