Potential tidy of answer here: http://stackoverflow.com/questions/27885264/creating-purescript-records-from-inconsistent-javascript-objects/28042483#28042483

gistfile1.purs

hush :: Either e a -> Maybe a
hush = either (const Nothing) Just

toId :: JObject -> Maybe Number
toId o = fromNumber <|> fromString
  where
    fromNumber = hush $ o .? "id"
    fromString = do
      s <- hush $ o .? "id"
      mfilter (isNan >>> not) Just (readFloat s)

toUsername :: JObject -> Maybe String
toUsername o = hush $ o .? "username"

toEmail :: Json -> Maybe String
toEmail o = hush $ o .? "email"

toIsActive :: Json -> Maybe Boolean
toIsActive o = either (const (Just true)) Just $ o .? "isActive"

toUser :: Json -> Maybe User
toUser json = do
  obj <- toObject
  id <- toId obj
  username <- toUsername obj
  isActive <- toIsActive obj
  return { id: id
         , username: username
         , email: toEmail json
         , isActive: isActive
         }

Thanks! hush improves readability significantly. I've incorporated your improvements into my answer.