"How many people have first hand knowledge of Jeremy Corbyn?"

The same question could of course be asked or Cameron, or May, or Gove or any other politician. But what is certainly the case is that Corbyn has spoken at more open rallies with 1000+ attendances than any politicians I have known. I have spoken personally to him, but only for about three minutes. Even so, that is 100% more than any other potential prime minister I have been asked to support.

As to how I form my opinions, it is based on his track record. Many commentators see a history of rebellion against the party leader. It is as easy to see it as a solid adherence to a set of principles. Now, whether that is realistic or idealistic is a matter of judgement on our part.