It would be weaker than a recommendation from an assistant professor or associate professor, but it would be better than getting a recommendation from someone who doesn't know you as well. Furthermore, since he's now completed the PhD, he can speak to your ability to complete the PhD as well.
When my students have asked me to write them a recommendation (I am a senior PhD student), I have referred them to the professor - but then again, I have never been the instructor of record for an entire class. If I were, I think I would tell my students that a recommendation from a professor/instructor with a PhD is better, but if they really want me to write them one knowing that anyway, I will.
As for the first question - if by "personal relationship" you mean this person was a mentor or research adviser who never formally taught you but did supervise or advised you in professional activities, then they could be an appropriate reference. If by "personal relationship" you mean this person was like a friend, or a a beloved aunt, then they probably are not a good reference just like any other friend would not be a good reference.