I've been using YouTube since early 2006. Since then, it has grown more and more popular and should continue in this way. It offers -lots- of videos, and I, as a musician, discovered many artists with YouTube. The videos' quality may not always be good, but well, everyone takes videos, either pro or amateur.
And, of course, "who doesn't know about YouTube"? To conclude, it's a really good site. I recommend it to everyone, BUT parents should watch out after their children when they go on the site because some videos are offensive to them.