Now, that being said, if we found plant life on another planet, then sure, I say colonize it. If we found evidence of another race, or some other form of aliens, that are intelligent, then no, stay far away from that.
Because where there are one, there are two, and where there are two, there are many. And those people might have some friends that we don't know about in another galaxy somewhere in the universe with big guns and wrinkled foreheads *cough*
I'd really not want to start a war.