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Yeah, the Drake equation is pretty cool. The only problem I have with it is it makes it look like we know more than we really do. We aren't positive what any of the factors really are, but most of them we have a decent idea.
For example, intelligence is something that has evolved seperately a few evolution. A large portion of intelligence research is focused on birds, because they diverged from mammals a really long time ago. Although they evolved seperately, they arrived at a similar place as many mammals. Many birds have problem solving and abstract thinking abilities, particularly parrots and the crow family. Dolphins are another good example. This leads us to believe that higher intelligence is a likely outcome.
But we don't know if human-type intelligence is common, because we have nothing to compare it to. It could be enormously rare, but we figure that it is probably somewhat likely.
Also, we don't know how many planets would actually make it past single celled organisms. Life first formed very shortly after Earth cooled, but we only had singled celled organisms for 3000 million years. It's only within the last 500 million years that we even had true animals. Perhaps many planets have bacteria, but it is extremely rare to have higher lifeforms. Who knows.
I guess the real point of the Drake equation is saying that any of the factors are kinda pointless. It doesn't matter how rare it is. The Universe is so so so so so so so so so so so big that other intelligent life is enevitable.
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