Music games, which are really the evolution of the original puzzle game model, has seen a real upsurge with the unparalleled popularity of franchises like Guitar Hero and Rock Band. The iPhone and, in a serious way, the iPad are positioned perfectly to continue this pattern because of the touch screen's ability to mimic an instrument.
Six String, from the Universal Music Group, is really just a clone of the same format as Guitar Hero where you have access to songs and you play along with them by matching your fingers along a guitar fret as colored objects slide along the strings. This requires you to get a flow down where you feel the general pattern of when to put your fingers down and how long to keep them there. Six String does this fairly well, even if it does not redefine the genre in any real identifiable way. This can be difficult to get down on an iPhone with its small size, but this is a challenge for those new to the iPhone in general. Once you are able to get the interaction down you will find that this can be a lot of fun.
There are some issues in Six String about how the music plays itself out, but as more tracks are made available this will become easier. For the full version of Six String you will pay $4.99 and get seventeen tracks, but then new tracks cost $0.99. This is a very clever way to sell music in an era where peer to peer file sharing has decimated the industry, so you have to make sure that you really enjoy Six String before you start sinking the money.