It definitely was for me too, and because it had _everything_, thanks to piracy. I started listening mostly mainstream bands, because that's what I knew, and within a few months I was listening to absolutely random authors, genres and "things" I didn't know existed. And by that I mean stuff like old sega genesis video game music somebody uploaded - without even knowing what the genesis was.

I never BOUGHT so much music since. It was eye opening for me how little I cared about radio/mainstream music after that.

The closest I've found so far is soundcloud. Thanks in no small part due to the barely-working recommendation engine, which doesn't get stuck in a local maxima, frequently going offtrack in pleasant unexpected ways.

Soulseek is still pretty good.