I really do love it. Great card, none of the whining ever gets to me.
As for "emotionally negative," its really a zero-sum game. Those times when you are possessed on your hand of all green/curse/copper cards, it is quite satisfying. Those times when your KC megaturn is being played out by your opponent, it is quite depressing.
However, that is only when being possessed. Don't forget, barring an edge case with Black Market, you could be doing the possessing and have the same chances of inflicting "emotionally negative" or "emotionally positive" turns.
So, in the end, Possession doles out emotional positivity/negativity in balance.
Also, I have found that the increased game time is generally negligible except in turns with insanely long action chains. However, YMMV.