For quite a long time now I've wanted to play Dwarf Fortress and every time I've tried the almost vertical learning curve put me off.
Until last night. Last night I put in the extra effort and it paid off. I only played for a few hours but I'm afraid I'm hooked. For those not in the know Dwarf Fortress is a Dungeon Keeper type game done in the style of a real time Roguelike. You take your merry band of dwarfs into the wilderness and attempt to start a new settlement there.
It has a user interface designed by the devil himself and gives you almost no guidance on how to play. There's a halfway decent first player guide here but it misses out on some critical information that caused me to need to restart 3 or 4 times before it clicked.
So it's hard, unhelpful, difficult and ugly but persevere and it's so very, very rewarding. Like Nethack and Roguelikes before it I fear I'm now a lifetime convert to the way of Dwarf Fortress.