I give the episode a C+. The science is stupid but I don't give a crap about science in Star Trek. In science fiction you just accept that all diseases can be cured completely over a few hours and DNA is magic shapeshifting juice.
I do agree though that there needed to be more reprocussions. Not from brains shrinking or things like that, but the fact that a few hundred of the crew should have been murdered and in some cases eaten by their friends. Worf should have spent the next week vomiting red fabric.
And ignoring the science, parts of the episode were staged poorly. That guy who got a claw across his chest? Why was he sitting at his console?! Even if for some reason he didn't run for his life when a giant animal crewmember was bearing down on him, how could the animal have clawed him from behind his console? And what happened between Worf getting a venom sac and Picard showing up? Apparently everybody panicked immediately, nobody sent out any kind of distress or warning signal, nobody tries to lock the ship down or anything to protect the crew from each other. They reacted with the competency of the first ship to get killed by the weird disease before the Enterprise found them.