Thanks to this thread, I picked up the game and it's incredibly addictive. There are some good mods - I recommend the Russian mod if you don't want to constantly travel back to town to replenish your army, and the Interface mod is outstanding.
Modding the basic data files is fairly easy. I just tested out one where I started with some very low-level demons (imps and a cerberus), just to see how that would be. Demons are fun, but they do have limitations that make them less-than-ideal as a starting army. Undead, on the other hand, are great once you get vampires.
Great game with an expansion coming out later this year.