When should I buy a Possession in Dominion?
Personally, I rarely design my deck to specifically target Possession, because:


6 coins plus a potion is difficult to get
I don't know what my neighbor will have


However, if I do have potions in my deck, and I happen to run across 6 coins and a potion during the game, I'll evaluate at that point.  Usually if the person to your left has any kind of reasonable deck (they're buying Golds and such) it will be worth it to get Possession.  Sometimes their deck is complete garbage and it's not worthwhile because possessing them will just clutter your deck with more Silver or other low-cost cards.

Remember, by buying Possession you're essentially sacrificing at least one Gold you could have had, so the hands you're possessing need to be of some minimum level of quality to make that tradeoff worthwhile.

Ultimately, I only get Possession in games where the other potion-costing cards make buying a potion worthwhile, and where if I don't happen to put together 6 coins plus a potion in a single hand, there's still a viable strategy.