For the record: This is the first iPhone game I played through, and pretty much the first casual game I finished as well.
The idea of the game is that you place a light source/torch inside a level and send out a blast wave from there that destroys enemies. Complexity is introduced by differently coloured enemies which require planning the order in which the enemies are hit and by enemies which are not passive, e.g. which try to destroy the torch after having been hit.
The concept gets a bit old during the last levels. I was constantly expecting levels which required more thinking outside the box, for example having to “sacrifice” one blast wave in order to finish the level later on, but there was nothing like that.
What worked really well was being able to play one one or two minutes at a time, so I often got the iPod out during small breaks or when travelling and played for a couple of levels.