It was really fun. 
 
The graphics were really good. 
 
The music was parallel with the atmosphere. 
 
I do not see anything wrong with the game, yet the demo was very short. 
 
The only problem I see maybe happening is it getting repetitive, but even in the short demo I unlocked 4 weapons and fought 3 amazing bosses and a couple mini-bosses. 
 
I would keep my eye on this one if I were you.