the right mix of classic recipes with originality
The game starts slow, but it gets better. The amount of different weapon configurations found in later worlds (spread, piercing, homing, backwards firing, ..) allows you to find just the right build for each level, which is a tactical element rarely seen in SHMUPs. And it's necessary, too, since the last levels are very challenging.
I've just killed the final boss and got the last medal, and I'm glad I kept playing after the first slow levels.