This documentary is bizarre and I guess you would have to have an American upbringing mixed with communist paranoia to understand it.
The documentary is supposed to be anti-Obama but it is paradoxical to the point of disbelief. It portrays Obama as having a secret communist agenda nurtured primarily by his supposed dad Frank Davis (who Obama no doubt did know). Yet in action during his presidency nothing could be further from the truth as most commentators on the left would attest to, Noam Chomsky for example. Numerous Islamic connections and sympathies are inferred in this documentary yet in action the Obama administration remains staunchly pro-Israel and carrying out more drone attacks in Afghanistan and Pakistan than Bush administration ever did.
In some respects I warmed to Obama during the course of this documentary because of the simple truth of wanting a redistribution of resources and equality in the world. Then I stopped myself and thought 'hang on the USA is still the home of the richest oligarchs who are getting richer and Obama put an end to the Occupy Movement' and of course then I woke up!
In summary, yes there are physical similarities to Frank Davis, the story of Obama's conception maybe plausible and there could have been a cover up. Yes the lack of a birth certificate looks odd. All Obama needs to do now is address this prove his heritage (if he can).
All in all even if he was the son of a Marxist, so what! And if that was the case I'm sure his 'real father' if he were alive might look on his reign with disapproving eyes!