Extraterrestrial life must exist! I would hope that some sort of evidence of life of some form will be discovered within this century, but actual contact with an extraterrestrial civilization will undoubtedly take considerably longer due to the distances involved.
I don't really think we should dwell on the possible dangers or benefits of a discovery of extraterrestrial life (reading science-fiction novels will illustrate most of them!)...it's just something that must be pursued, even just out of pure curiosity and thirst for knowledge.
I believe that the ultimate aim of human civilization has got to be to expand beyond the boundaries of the earth and this solar system, otherwise our long term future is probably very limited.
I run SETI@home on oldish computer(s) at home and, when I can, on my workstations at work - they're far faster than my home PC and are otherwise sitting idle for 16 hours a day on weekdays and all weekends...just got to use all that wasted processing power!