Once you understand the sheer vastness of the universe and the staggering numbers involved when considering not only the number of galaxies but the number of stars in each galaxy.
Then factor in the abundance of organic compounds out there and the fact that once life does start how evolution takes over to produce some extremely resilient lifeforms.
Then consider the unimaginable amount of time involved. The Universe is over 13,000,000,000 years old. Many of the first stars where huge monsters that exhausted their fuel in just a few hundred million years after which they exploded spreading elements all over space so the ingredients for life have been around for eons upon eons.
You also have to take into account that life does not have to start on 'Earth' like worlds. In our own solar system we have many candidates for life. Mars is obvious but there are also worlds such as Titan, Europa and perhaps even Jupiter itself. Jupiter, I'll admit is a bit of a long shot but if all the necessary ingredients are there, who are we to say what nature has realised.
There is also the theory of panspermia or exogenesis which states that the seed of life has been transfered to the Earth from elsewhere in the Universe. In our case most probably asteroids and/or comets. If this is the case then it is quite possible that life only has to evolve once to 'infect' many different worlds.
It now seems that not even a month has to go by without a new planet being discovered and this now points to the notion that most stars have some sort of planetary system. The Drake equation is getting close and closer to being solved. Even though most of these planets seem to be of the Hot Jupiter type it is only a matter of time until the techniques are refined enough to discover Earth like worlds, in fact a planet of only 5 Earth masses was discovered around
Gliese 581 and that's with the technology we have now. Imagine what we may discover in the years to come.
As to what kind of life exists out there. Who knows. I think that given the right set of circumstances and the possibility of Earth like worlds then evolution will shape life in quite a familiar pattern. If animals as we understand them are to evolve then they will obviously need to get around, in which case some form of limbs are going to be needed. Also, eyes have evolved independently on our planet at least 5 times so it seems quite reasonable to assume the same for other worlds.
For worlds unlike the Earth then that's anyones guess. As mentioned before there is no reason why the chemistry of life can't take hold anywhere it has the chance. As Arthur C. Clarke said, I can even imagine some form of life existing in the outer atmosphere of some stars although whether nature has realised it is another matter.
Intelligent life however is not the pinnacle of evolution. It is simply one of the many solutions for life to exist and succeed. But you must admit it does seem to be working pretty well, so far, for the human race and so why it can't work on other worlds as well I have no reason to doubt.
I have a feeling that like life itself, once intelligence and civilisation takes a foothold it is practically impossible to remove. And given a race that has managed to shed it dependence on its home world and has spread out not only to the rest of its solar system but also developed inter-stellar travel I think it would be remote beyond belief.
I'll go even further and propose that a race that has been spreading out like micelium for millions of years would become so fragmented and taken such diverse evolutionary paths, both by nature and design, that most of the now sparsely distributed colonies would effectively be different species from each other. I can imagine entire galaxies, or at least vast sections of them, populated by a vast array of different alien species each with their own worlds and ecologies that seem very different from each other but can all trace their ancestry back to one world many millions of years ago.
So, given all that the proposition that the Earth is the only living system in the Universe is, to me, as likely as the Earth being just 8000 years old and all the dinosaur fossils being placed here just to trick paleontologists.
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