I heartily agree with CaptainX, Sam Snead, and dojetdriver. Having been a captain before, and now being back in the right seat myself (hopefully not too much longer), I've seen both sides of this fence. I'm sure at many airlines, there aren't many experiences more unpleasant for an FO than flying with a "New Captain."
People bash Great Lakes a lot on this board (and much of it is deserved) but they turn out some very fine captains there. Yes, even new ones. Why? Because they start training FOs to be captains from Day ONE. I know there are some things can only be learned through experience, but with the right corporate culture, there are a lot of things that CAN be passed on in a meaningful way to a pilot while he is in the right seat. Obviously, this doesn't happen very much at any airline where the prevailing attitude is "FOs are stupid."
The time to train a captain is BEFORE he upgrades so he doesn't freak out when he gets in the left seat.