Well, I don't really trust random certs even when they're signed by a respected CA -- but I still prefer using HTTPS. Even if the cert is fraudulent, HTTPS is still encrypting stuff and will protect me from other random attackers.
Security is never a binary secure/insecure proposition. There are shades of gray. The key is to use what security you can, but never think "I'm secure now".
As an old mentor once told me: the moment that you think you're secure is the moment that you're at the greatest risk, but you should still lock your door.
