It's one of the few 18+ games available on the PSP, though I personally don't understand why, as graphically, you don't see blood, swearing, drugs, or anything but hack-and-slash. However, enough about that; what makes this game good?
Well, let's see... It is by far the best graphical game out there on the PSP, and by all means, a very long game, if you consider the modes you can go in, so this would keep you quite happy for the odd play on the train back home. It is a must have game, which is already known, as it has become Platinum on the PSP (I have the original version), but makes it even better, as you get the game cheaper than what it's worth in context!
However, I don't give a 5 star rating, because there isn't a large variety in the game. There is only one weapon to use, which can get repetitive a little. Nevertheless, a great game, and at this price, surely it is worth it?