The two arguments i hear from speculating astronomers and astrophysicists and theoretical phys. are that either a) Such an intelligent species (probably more intelligent than us assuming they come to us first), would be wise enough to understand how precious life is an would have no intentions of causing harm. They likely would not need any of our resources since they are abundant throughout the universe. My personal spin is that if they were going to kill themselves with war (like we are definitely going to), then they would have done it already and thus we would not be meeting them.
The other speculation i hear is that we will be so insignificant to the other race that they would pretty much either conpletely ignore us, or would have no problem just kicking our ant hill over for fun. I think we are classified as a type 0 species, and a species that can travel between stars and harness the power of the galaxy would be type 3. We would likely encounter a type 3. Is a type 3 going to care about type 0?
...i would say maybe; it depends on how abundant life is in the universe. If we find bacteria on Europa, it's not like we're gonna say "UGH, germs!!! What a waste of time!" *busts out the purell*
...no! If we find bacteria, it's going to rock our socks off. Even though our relationship to bacteria is likely proportional to a type 0 and type 3 civilization.
So my verdict is, yes, it would probably be a friendly encounter. Just remember - if a civilization is so advanced that they can travel between stars in the first place, they PROBABLY got their s#*t together reeeeeal good lol. There would be lots of order in their civilization. They probably wouldn't be a bunch of rogue barbarians. And even if they were, we don't have anything that they would want anyway. If they come to our planet, it would probably be for the purpose of discovering life. They would probably have experts who know exactly where to look. Even we are starting to identify potential life-harbouring extrasolar planets, and we have only recently left our own solar system for the first time.
So how would HUMANS react to this? We would probably either kill ourselves or try to kill them (and then be killed by them in response), because we are a bunch of monkies.
Another thing to keep in mind is that up to this point, Earth-based life is ALL we know. That means if we ever did encounter ET life, we may not recognize it as life, and that's something we have to be aware of and anticipate. Life elsewhere could be based off of COMPLETELY different principles. They might not be carbon-based. Just here in Earth, we see creatures that look alien to us. Imagine a species that evolved separate from the very beginning. What if it doesn't even meet our current definition of life? Would we be flexible with that? If they are not "living" by our definition, does that make it okay to enslave them? I can see how it might get complicated when it does happen.
I think District 9 is another interesting outcome.