Yes, I believe extraterrestrial life exists.
When will we discover that other life? Not for awhile is my guess. I don't think we've found any yet, to be skeptical.
In addressing the question of "What are the dangers and benefits..." I reply with, "information wants to be free." The dangers and benefits of finding that life is secondary to knowing about its existance.
Some people will be scared, others will be overjoyed. It doesn't matter.
Yes, we should have some sort of calling card floating around out there. As far as what information to include... basic stuff, I guess. If they're smart enough to detect the signal, they'll probably be able to find out where it came from, visit us and figure out the rest.
I run SETI@Home because I think the project has some ambitious but admirable goals. They're long-term. Maybe we'll find something in my lifetime, maybe not. In this case, the journey itself is not the goal ...but it is a lot of fun.
One last note ...I restarted because of a Geocache that a friend and I recently visited. A fellow Geocacher here in Oregon made a steganography and SETI-oriented Geocache. One of the notes he included was that individuals searching for the Geocache were encouraged to get into the SETI@Home project. So I did. | 
