I watched this week a few friends, honestly, it had me hooked on the first part.... we theorized things like "What if the girl outside is a hallucination?" or "Are the bad guys really the bad guys? Is it actually the treatment?" and all that. The intrigue I had was unmistakably there, though they just kicked the original vibe out of the window in the latter half.
The first part felt like something that could be fleshed out, besides the strange things to note like "why aren't there any anesthetics when they do the procedure?" and "wouldn't the carbon dioxide be a 'contaminant' in the air?" otherwise I thought it was interesting, but didn't feel like I could find the message just yet, and then there was the horror scenes which I found actually good.
But now to the latter half... it felt like it was just suddenly injecting a weird cult plot in there, and Eli being the son of the devil and never actually having a sickness felt so different from the first half of the movie. And then also making the girl be a devil kid or whatever. Kind of cheesy to me.
TL;DR: The first half felt like a psychological horror that felt actually down to earth, and then the sudden switch to a supernatural plot felt out of place. Overall, the movie felt like stitched patches on a fine coat.