I believe the people who are giving this game low ratings are holding it to an incredibly high standard. That is unfortunate because the game really does have a lot to offer to PSP owners.
The story is interesting, and qualifies as unique. The action is fast and challenging enough to give players a sense of accomplishment when they beat a level. If you want to compare it to other games you've played, this is best described as a cartoony Devil May Cry.
The only flaw I see in the game is camera and control. Yes, this game would benefit from the two stick dual shock control, but that's just not an option. I think a lot of people have gotten spoiled to the dual shock and thus have a hard time adjusting to the more simplified PSP control system.
Overall, I think this game is a great addition to the PSP library and hope to see more original games of this nature appear on the system. An enjoyable experience and worthy of purchase for PSP owners.