I believe that extraterrestial life exist, because universum is huge and it would be childish to think that we are only intelligence race around or even only race. I dont believe that there are alien ships flying around the earth (even i am fan of X-Files ;), no I dont believe they ever visited us...YET! Hard to say when first contact will happen, but I think it cant be far if we continue projects like SETI@home with this kind of volume. Benefits...hmm...well, they could be more advanced than us, so we might get new technology, but of course there are possibility that they are hostile toward us, but like we say in Finland, "that will not play, who feels fear" and after all, first contact will be biggest step that humankind has ever taken in history.
I think humans should do everything to contact aliens, so sending beacons to space is good idea, but havent really thought what kind of information beacons should send out there.
Personally I run BOINC cause it's probably most important job my computer has to do.