The songs will not be compatible with ipods until march/april time. Also, the tracks are free but they come with advertisements, which is how they can afford to pay the record companies. They have not released (to my knowledge) any information on how many ads you get per song, what sort of ads they are, how intrusive they are etc.
Say for example, if you have to listen to or watch an ad every time you want to to play the song, most people would find that annoying as hell. However, if it's more along the lines of the ad being shown while the track is downloading, then I can see it being popular.