I liked this movie a lot. It reminded me of Primer in many ways. Both sub-90 minute low budget, high concept science fiction with very few special effects. In Coherence the only effects were tricks shots to make it look like there was two versions of the same character, and I guess some shots of the comet which may have been archive footage. Both movies did a lot with a little. All movies essentially use resources to manufacture drama, and Primer and Coherence both craft great movies out of almost nothing. Coherence was just a small group of actors, a camera person, two sound people and a director in a house. The reported budget is $50k which is presumably the combined salaries of the cast and crew.
The main differences are subject matter, time travel vs alternate realities, and Coherence being about the interactions between a group of people, whereas Primer only really has lead characters. I'm not prepared to say that Coherence is as smart as Primer. I think the differing way the two films were made is related to this. Primer was as far as I can tell planned out in great detail and executed to perfection by Shane Carruth and a few other people. The process of making Coherence seems as if it was more experimental in nature. I didn't know about the improvisation and the actors having notes with their goals for the day instead of a script before watching, that's fascinating to me. I'd be really curious to read their note cards and otherwise find out what exactly the plan for the film looked like before shooting began. I wonder what filming was like. They just rolled camera and acted out the dinner party while incorporating their goals? I assume they must have had some things they knew they were supposed to say as part of their notes, such as Emily telling the story of the comet from 100 years ago, and tall guy reading from the exposition book. The actor playing Amir was one of the writers and would guide the other actors sometimes apparently, so I want to watch it again and pay attention to him to see how often he says something that moves the plot forward.
edit: one part that I though was hilarious maybe unintentionally so was when Mike says to tall guy something like if there's endless realities I slept with your wife in every one of them. Mike was a good character in general.