DF is immensely fun and complex. Sadly late game lag and a steep learning curve keeps the fun from getting through.
Honestly dwarf fortress offers something for everyone. Dwarfs are known for their creativity, stupidity and stubborn alcoholic small bodies. Dwarf fortress allows you to capitalize on all those traits. Build pretty much whatever you can imagine from simple goblin traps to extremely complex binary logic computers to extremely pointless marvels of dwarven engineering like a royal tomb carved out of obsidian your dwarfs formed in the middle of a volcano. The game offers city-builder standards like logistic issues between workshops, housing requirements, and happiness problems among your stout citizens.
The most noticeable thing is the 2D ASCII graphics in the game. Some people may find this a little lacking but surprising it adds much to the game. ASCII graphics allows the player to imagine the world in a way that a 3D game never could.
Dwarf fortress is a fun game full of humor, complexity and customization. The game has a steep learning curve that is offset by an excellent wiki. The game has both a city/fortress builder and an rogue-like dungeon crawler that allows you to explore your old fortresses. DF offers good times for a long time plus its free.