Pandora’s internet radio has always been one of those sites that was really cool in concept, but too inconvenient to ever go mainstream. The service was long tied to computers only, and while it eventually expanded to special internet radios and some mobile phones, it still has yet to become a household name. But with the launch of Pandora’s new iPhone app last Friday, it looks like the service is about to hit critical mass. It’s a free, mobile, digital radio station that only plays music you like and lets you skip the stuff you don’t. And it rocks.

The personalized music service employs a small army of 50 musicians to create a “Music Genome” that describes each song according to 600 attributes. Listeners input a few of their favorite artists, and the site analyzes the Genome to serve up an endless stream of recommended music.

We introduced the app last Friday, when we called it our “flat out favorite application so far”, and since then it hasn’t failed to impress. Streamed music plays flawlessly over Edge and 3G networks - during a 40 mile drive I didn’t once run into any kind of skipping or static. Even better, the app currently has no advertisements playing, though we can probably expect that to change.

Unsurprisingly, Pandora’s usage stats are overwhelmingly positive. Pandora is currently the fourth most popular free app on iTunes (behind Apple’s Remote, AIM, and WeatherBug), and has reportedly been seeing a new listener every 2 seconds. Usage over the weekend hit an all-time high for the service, with 3.3 million tracks streamed to iPhone listeners alone. Perhaps more impressive is the retention rate of listeners, who are averaging over an hour of listening per day.

If there’s one thing that could kill the service, it’s ads. Pandora is going to need to monetize the app somehow - let’s hope it allows us to pay an upfront fee (say, $10) to avoid the annoying interruptions that have made listening to traditional radio a painful experience.

[techcrunch]