None can take both 'is' and 'are', depending on the context. I am not sure that there is one, correct answer here as it is open to interpretation. My inclination is to use 'are' in the first sentence, as there is a sense of plurality. But in the second I would use 'is', because here you are comparing one individual against a group.

None of them are useful Because them is plural, which needs 'are' when referring to them None of us individually are as clever as all of us collectively

The online Oxford Dictionaries states: It is sometimes held that none can only take a singular verb, never a plural verb: none of them is coming tonight rather than none of them arecoming tonight. There is little justification, historical or grammatical, for this view. None is descended from Old English nānmeaning ‘not one’ and has been used for around a thousand years with both a singular and a plural verb, depending on the context and the emphasis needed. So we can use both interchangeably depending upon context....

None of them are useful (because none of them is plural, that's why you use "are") None of us are smart (same thing again, "are" because none of us are plural) Hope that helps :)