When someone looks straight ahead to you, this is usually a good sign. After all, most of us will try to catch someone's eyes by looking at him/her to show our interest.
But do consider that people usually also look away as they're processing what you're saying. So, if someone looks right at you, and doesn't look away at all, the message of what you're saying may not even come across. Also, people who are lying are often aware that eye movement may give them away, so they'll look you straight in the eye. And keep looking.I guess that's why it’s usually a bit creepy if, that prolonged stare in the eyes.
In short: Straight at you... good, continuous looking without signs of looking away... often not so good, disinterest or lying. Exception: the gaze of 2 people deeply.