I think there is a lot more to this question than just polling people's opinions; there is a large body of work out there on the subject of astrobiology, and it is becoming more systematic all the time. Thanks to the catalogue of extrasolar planets now available it is possible to put some real data behind these speculations.
It is something that I. for one, have been thinking about all my life; I really wanted to do astrobiology at college, but it wasn't available- in fact I studied environmental science, always with an eye towards the conditions on other worlds. In fact I went to the same uni as David Darling, who has become something of a specialist in this field- he left the same year I went there, so we never met.
So astrobiology is certainly speculative, but there is a certain amount of data available which can be used to form opinions. This amount of data can only increase.
If you want to speculate about intelligent and/or civilised life in the universe the field of study becomes much wider, and perhaps more interesting, but also more open. You need to know a little about anthropology, and world history, and the science behind the development f sentience. For instance take Wildmoon's question;
Why does life out there have to be either highly advanced or microbial to everyone? Why not neanderthals? Why not people as advanced as us? Why not civilizations similar to the Greeks, and the Romans, and the Egyptians?
is a fascinating question, and some rough estimates can be made of these chances.
Sun-like stars have a wide range of ages, and few re exactly as old as our own Sun; this suggests that life bearing planets ill have a wide range of ages too, and will be encountered in a range of stages of development. We have the example of our own world to guess what those stages might be.
Since the expected lifespan of the Earth is about nine billion years we can estimate the chances of our encountering a similar world amongst the subset of Earth-like terrestrial planets with life.
For a couple of billion years the Earth had almost no life, and had a reducing atmosphere. We are likely to find quite a few of those. Next the development of photosynthesis led to a cyclic alternation between oxygen rich and oxygen poor conditions, caused by the fact that oxygen was toxic to the very organisms which produced it; this period, during which the Banded Iron formations were laid down, lasted a little less than a billion years. Once oxygen tolerant organisms developed the atmosphere became increasingly oxygen rich; a slow process, as the crust hungrily absorbed much of the free oxygen.
This state lasted more than a billion years until the Cambrian period, when multicellular life developed. This latest phase, an Earth with a complex multicellular biosphere, has lasted half a billion years- about 0.6% of the Earth's entire history so far. But we can expect this rich biosphere to persist for perhaps another billion years or longer, until the Sun warms up and causes the Earth to develop a wet greenhouse climate. So, barring accidents the period with a complex biosphere could be the longest of all the Earth's stages.
Any random Earth-like planet we meet might have as much as a 30% chance of having a complex biosphere (I am only considering worlds orbiting G-class stars for the moment, by the way). The last three billion years of the Earth's existence will be increasingly inhospitable, but life may adapt to these conditions; we must expect to find many such marginal worlds.
Now what are the chances of encountering a semi-intelligent species like the Neanderthals, or a pre-industrial civilisation like the Egyptians?
The Neanderthals as a species only existed for a million years or so; if Homo sapiens sapiens had not wiped them out they might have survived for longer. An average species lasts about two and a half million years; but to be generous we could allow them to last as long as the other semi-intelligent inhabitants of our planet- the dolphins. Around thirty million years or so. Given a random earthlike world with a randomly chosen stage of development, we might expect to find such a semi-intelligent species on [30 million divided by 9 billion] = one third of a per cent of all such worlds.
Pre industrial civilisations are even more rare; the earliest cities re dated to around ten thousand years ago, so a pre-industrial civilisation might last ten thousand years before becoming industralised; the chances of encountering such a preindustrial state might be [10,000 divided by 9 billion] = 0.0001% of all worlds.
It seems possible that such preindustrial societies will be very rare..
