Adding more floors doesn't mean more fun. The last five floors are a real endurance test of your equipment vs. the permanent stat debuffing mobs. I had to skip 2 floors because my equip was getting worn down so bad. A buffed Dredmor waits at the bottom of Floor 15 but he is unreasonably hard. I think the devs expect you to cheese him with throwing weapons considering how the throwing vending machines from 11-15 are full of holy hand grenades. If you are considering getting this for the new skill trees, don't bother; they are very niche, often doing more harm to you than the enemy.
This expansion adds a ton of infuriating content and is by far the worst DLC. Its saving grace is that you can disable the Diggle God floors when making a new character. Pick up Wizardlands instead.