Worms 4: Mayhem just isn't worth the purchase if you've owned the other 2 3d versions.
The only addition over the last 3D version is the ability to customize your own weapons. Other than that the game remains VERY similar to the last.
The controls remain clunky and unresponsive, and the camera is still a pain to control. Jumping is still a difficult task and really takes away from the game.
The gun customization is really the only thing good about this game, and therefore I highly recomend not picking this up.