I remember loving the word "endosymbiosis" when I learned it in my 9th grade biology course. The idea that two organisms could cooperate for the benefit of each other was enchanting; it parallels the obsession our culture has with love, as well as the hope that we human beings are in fact more than the sum of our parts; we hope Newtonian physics isn't all it's cracked out to be. What made endosymbiosis so compelling, though, was that the two little critters which made up lichen, a blue-green algae and a fungus, were dependent on each other that the removal of one would cause the other to die. It's the (very)pre-cambrian Romeo and Juliet.
But we must concede that not all interactions are this mutually beneficial. We see ranges from parasitism to mass enslavement. Parasites take all the work and energy of another individual, and don't seem to mind bringing hosts to the brink of death to avoid having to evolve anything useful.
There are ant colonies who enslave other ant colonies and put them to work for the rest of their lives. If you had a microscope, you'd see works much greater and more impressive than the pyramids. What I'm trying to say is that, well, it really depends. When organisms are involved in things together, only in rare cases do they work out, but when they do they really flourish.