That damn game...
Such a terribly wonderful time-sink. I agree that anyone with a cartographic bent can find much to enjoy there. If the ASCII graphics put you off too much, there are some great tilesets available that add a little more personality and readability to the display - such as Mike Mayday's lovingly rendered and compiled version found here - http://mayday.w.staszic.waw.pl/df.php
I can honestly say that this game taught me a few things about dungeon design, and emphasizing a connection between form and function.
Requires a bit of a mental commitment though, as the learning curve is a sheer 90 degree cliff face for your first couple of fortresses. Praise be to the wiki.