First, you can listen to 30 seconds of all their songs for free.
This is the cool part of the service:
Basically, search for what you want, listing to it (if you've never heard it before or aren't sure if you selected the correct song). Once you know what you want - go load up Kazaa and download the full song.
iTunes is a great little app to browse and sample music (its like going to Best Buy where they have those headphones set up to listing to some CDs).
Of course, if you can afford it, just pay the $0.99 for the song you want. I don't think I would pay unless the songs are about $0.49 each.
Searching for "Gravity Kills" returned Zero results, which pissed me off. There are two CDs of theirs I was considering buying soon, and I wanted to see how easily I could get one song for $0.99 (to test the whole "buy a song download" concept, and see how their format works).