Fido Full Review 'Warning Spoilers'
I think one of my main problems with Fido is that it plays fast and loose with the conventions of zombies. At the end of the movie, Fido is basically a fully functioning member of the family, with the exception of being a little stiff. Even the collar is not really necessary. For a movie like this to really work, I think you need to respect the rules. Shawn of the Dead managed to work within the rules of the zombie genre. Consider a somewhat similar relationship in the film Zombie Honeymoon. In that film the husband slowly devolves into a zombie, and his wife tries to stand beside him. Another example is something like Braindead (aka Dead Alive), where Lionel is trying to cover up his mother's zombiness.
I think the writers should have kept Fido a real zombie. When the collar is off, he is viscous, but with the collar on he is docile. I suspect that part of the problem is that the writers did not respect the zombie genre enough. Another possibility is that they did not want to make Fido as dark as it would need to be for that to work.