I decided to get this for my iPhone, as there's no music whatsoever on it until my new car head unit turns up and I have a spare 4 hours to filter through what music I do and don't want. There's a paid for version and a free version. If you literally just wanna listen to music, and don't care about much else, just grab the free one. There's a few features, but nothing I can remember being particularly wow. So I searched for metal, and browsed for a decent radio station. It brings up loads from all over the world, since it's internet radio. Choose a station, give it a few seconds, voila, instant music.
I have to say, this app is worth the download if even just for Onslaught Radio They literally do only play metal. Their wittiest remark so far, "Onslaught Radio, so metal, we shit studs"