Personally I think it is beyond question that life exists outside our solar system. Due to the sheer volume of stars (granted, not all of which hold a planetary system) there must be countless planets out there on which life could exist. It does not have to be (to quote some quasi-famous doctor dude) "life as we know it."
There could be organisms in the methane seas of Titan for all we know.
Of course in order to develop into a more complex species (like ourselves) the planet in question must be able to allow the development of a wide range of species; a food chain for want of a better description. Man couldn't survive on this planet if there were no animals and plants (even though we seem to be trying hard to disprove this by ridding the planet of these things!), much less have developed into what we are today. So I guess that makes the required conditions a little more unique.
Another aspect is whether you believe in faster than light travel. (With regard to contact rather than existence). No matter how advanced a species may be, if we can't travel FTL than visiting another intelligent species will just probably never happen. (Unless we're interstellar neighbours, which is possible, but still a logistical nightmare. We can't even get a single human to Mars much less to Alpha Proxima).
Of course many other intelligent species may have come and gone already. Lots can happen in 10billion years, and I don't reckon mankind will be lucky to last another couple of thousand.
And a final thought: Are we debating the possibility of intelligent life existing only within our galaxy? Cause there are more galaxies out there than there are stars in the Milky Way and Andromeda combined. Some of which make our little spiral look like quite insignificant.
Interesting thread.
TF Figs of 2024:
1) Legacy Magmatron
2) Legacy Gears
3) SS102 RotB Optimus