Ambiguity at the end of movies can be very good. Ambiguity at the end of short stories I am more ambivalent about; I would have to take that on a case-by-case basis. One thing that absolutely does not work in movies or short fiction is internal inconsistency. Sadly, some recent movies may not be internally consistent. While enjoyable, The Adjustment Bureau (also influenced by Philip K. Dick's 1954 "Adjustment Team"!) doesn't seem entirely consistent. Do people have free will or are they controlled by The Adjustment Bureau's plan? I don't want to give anything away but if this plot was submitted to ElectricSpec, I'm not sure it would succeed.
A more serious consistency issue may be associated with The Source Code. Again, I don't want to give anything away because this apocalyptic Groundhog Day is enjoyable. But it's unclear if activating the source code brings the protagonist in the past to an alterate reality and/or an alternate timeline and what reality/timeline the source code affects. If there are different timelines/realities how can they interact and affect one another? As you may guess from my remarks, this one's a real mind-bender. I would have to say I would've rejected this plot if we'd gotten it in the slush. (If anyone understood the movie, I welcome your comments.)
My advice to ElectricSpec writers is: Make sure your stories are internally-consistent!