Rage HD shows that first person rail shooters are the way to go on the iPod/iPhone.
The graphics in Rage are probably the second best I have seen so far. It's nowhere as good as Infinity Blade's but is still a beautiful looking game.
In Rage HD there isn't really a story, other than that you are on a TV show, and your job is to shoot all of the mutants that you see. The announcer provides a sometimes funny commentary while you shoot and fight your way through the level collecting money, ammunition, and killing mutants.
Overall I believe this is another essential game to have on your iPod/iPhone if you are a gamer, and I don't believe you will be disappointed at all with your purchase considering it is only $1.99 on itunes.