I was in Intermediate level of English by the time I joined here. ATRL certainly helped me improve my English, and acknowledge of English slangs I have never seen before in my life. To give you an example, back in that time I didn't even know what IKR? or LOL meant.
It also allowed me to meet people from another parts of the world - awesome people - and actually communicate with them. I consider this as invaluable, especially since I never traveled to another country and I probably won't do it until when I become financially independent.
It also broadened my musical knowledge to levels I couldn't not have even imagined, and sometimes is awkward because everytime I'm hanging up with my friends and I hear a song I know, I tell them the artist, name of the song, album, and the year, though I'm starting to find it cool now. :P