I believe the correct form is "None of us is perfect". However, since none of us is perfect, some of us tend to use the plural instead. Both would be easily understood and neither would raise an eyebrow.

"None" is a third person singular noun. It is the contraction of "not one". So when you say "none of these cars is good" you are saying "not one of these cars is good". The subject of the sentence is not "the cars", the subject of the sentence is "none" (or "not one") thus, it must be singular. As for "some" (mentioned by Charles), this is called partitive, it is used to show "part of something", and it can be plural or singular depending on the noun, "some coffee is good in the morning" or "some cars were parked outside", if the noun is countable the verb will be conjugated in its plural form, if the noun is uncountable then the verb will be conjugated using the singular form. I hope this helps.