Hard, but that's what makes it lasting.
Let's review this like a good...well, reviewer.
Difficulty: Freakin' hard. Can make some whine at the difficulty or just F-rage. But it's supposed to be a challenge, as it's not impossible.
In-Game music: Good. Fitting.
Graphics: To be 3D, sweet.
Gameplay: Easy to learn, but each weapon's rather tricky to master.
Lasting Appeal: depends on the player. For those who love challenges, this is one sweet game. For the ones who like easy games, ehh...Not so much.