Got like 200 hours into it. It's a really fun game, and really rewarding too. It's definitely challenging in the beginning, and there are additional challenges (like secret floors, rat boss, golden dragon) to entertain the experienced players. Once you've mastered the game however, the runs get very boring, due to their length. It takes like 45 minutes to do a full run including all of the challenging content, and once you've beaten the rat on floor3, you get such an insane powerspike you win the game from there.
Out of all the 'Isaac clones', this one's pretty damn good!