Installation and registration for Pandora is quite simple and you'll easily be able to enter the names of artists that you like, and it automatically generates a station to play you music it thinks you will like. You can "thumbs up" or "thumbs down" individual songs to help further refine your music recommendations.
TechCrunch reports that Pandora's iPhone adoption is impressive. A new listener has been added every 2 seconds and 3.3 million tracks were streamed to iPhone listeners this past weekend. The music stream works on EDGE, 3G, and Wi-Fi networks.
Pandora is free and available at the App Store.