I would presume that there is only enough room for a single sufficiently intelligent species on a single planet. The Neanderthals were arguably on a similar level of intelligence to humans, but with the introduction of humans to their habitat, they went extinct. This may or may not be a direct cause and effect, but one way or another, it lets us speculate that the niche is actually a very small one. From the history of humanity, and the level of (evolutionary fell-founded) xenophobia, I would say that even if at some point in relatively modern history there would have been more than one more or less equally intelligent species, their cultures would have conflicted constantly, eventually driving all but one of the species extinct. This, of course, is nothing we could extrapolate to other planets and possible life-forms, but the conclusion would still be that the fact that out of millions of modern and extinct species, only one has evolved to a high level of intelligence doesn't allow us to make predictions about the rarity of intelligent species on other possibly life-inhabited planets. All we can deduct from our planet is that life, once it has come to existence, has the ability to spread over the entire planet and transform it completely. Also, that consciousness is a possible outcome in the evolution of a nervous-system or its analogies. How likely either of the events (processes) is, is up for speculation, but the fact that life seems to date back to times where conditions that could support life were relatively new, would indicate that abiogenesis isn't a very unlikely process, although based on a single example, the conclusion may not be very good. On the other hand, we can follow the evolutionary path and see that the development of a nervous-system has proven to be a rather good adaption in more complex animals, but this of course may be just the only realized option out of many others.