I've joined many sites like these, usually nationality-oriented and tech-specific (forums for PSP wares or symbian phone apps, for example) and I say they're quite useful. The risk though, is huge. They do often say what Holo just said, "we don't actually host any of the files here" but some were closed down anyway. I don't know how, maybe they just went out of cash (but that's too improbable) but one day they just went poof.
It would be interesting though, to have a sharing-forum that is full of anime-oriented people. I've never seen one. I've been to forums where people do share anime, but it was never the main focus of the site. It was always the side dish. Hence when I ask about <insert anime other than bleach or narutardz here>, they go "wtf is that?"
