It adds monsters that will attack you! It makes cave-delving so much more challenging and interesting.
Most of the interesting and challenging monsters spawn very deep down. The surface monsters are not very dangerous. They don't impede surface construction or exploration. That might be a good or bad thing, depending on the kind of experience that you want.
One of the few if not only existing author that takes their mods seriously to ensure critical bugs are fixed promptly. If your a serious server admin like me, you can't go wrong with their mods' collection and the fact that prompt support is normally provided to boot. :)