I'd give it a B. There's a lot of good performances here, particularly Lupita Nyong'o's darker role as Red. I can see where some people took issues with the comedic elements, which aren't inherently bad but could be seen as detracting from the horror elements.
My main complaint has to do with the last act of the movie, where Red describes her theory as to how the Tethered originated. It's got some interesting elements on paper, but the problem is none of them really piece together in such a way as to make a coherent picture. Red is arguably a rather insane and unreliable narrator, but that too could have been used better. Supposedly, the Tethered are the remnant of some secret government project designed to clone people, on the premise that the connection between the original and the Tethered copy would allow those behind the project to manipulate the originals by how the Tethered behave.
This is a pretty cool concept, on paper, but it's not really clear if that's the correct case or not. Red speculates that the project failed several decades ago (generations even) and the Tethered were simply abandoned underground, to spend their lives blindly mimicking the actions of their "progenitors." The movie never explains how this connection actually exists between the two or the extent to which it works, and I wish that had been better utilized. It's implied that the environment and behavioral conditioning they're exposed to can be overcome in time, as happened with Adelaide after she swapped places with the real Red (her counterpart from the normal world). I'd also infer that, since the Tethered age in real time with their counterparts and no one seems to have more than one copy, then any genetic samples would have been acquired by the government when the person is an infant.
If the Tethered were really and truly abandoned by their "creators" in the project, then why are the warrens they live in apparently well supplied with many of the things necessary to sustain them (power, clothing and water, for example)? The movie implies they eat nothing but rabbits, which isn't the best diet, but they presumably can't supply themselves with food because they're locked into the connection of what their other selves are doing. And if the project officially ended long ago, why are the Tethered we see copies of the most recent generations (implying that someone is at least still copying the genetic parameters necessary to make Tethered)? These are all interesting possibilities, but the movie doesn't really do anything with them. And it's a little vague why Red didn't try to escape, before she presumably accepted some elements of the conditioning after the switch, or how the dancing apparently helped her find the "divine" mission for the Tethered. It would have made a bit more sense if she and Adelaide developed a shared talent for singing (IMO), since Red seems to be the only Tethered who can speak to some degree. That would have certainly made her seem special to the others.
Overall, it's a pretty good film. I've read a lot of comparisons to Peele's other film, Get Out, but I can't offer a comparison because I haven't seen that one. I just wish that elements of the Tethered were better thought out for the origin given in the film, and maybe it would have been better to have them as a more supernatural element (as many inferred from the initial trailers).