* What decks like Talisman?
Engines that need <4$ cards, Slogs maybe to get a lot of Silver if there is no better alternative.
* Would you ever open Talisman?
Yes, when I need a lot of <4$ cards. I think when Talisman is good you need to open with it to get the extra gains as soon as possible.
* How does Talisman compare to Duplicate? (hypothesize away!)
I think Talisman is much weaker. Duplicate is not limited to Treasures and Actions so it can gain extra Duchies, it is not limited to buys so you can duplicate HoP gains or Border Village gains (probably only relevant when you buy the last Border Village). You can chose when to duplicate your gain and do not need to play Duplicate the turn you want to use it. However, Talisman is non terminal and it gives a (very weak) bonus when you play it, additionally you can use it to gain extra cards multiple times per turn if you have +buy.
* Would you consider Talisman a good card? Why or why not?
I consider Talisman a weaker card as it is very board dependent. Usually you need to get 5$ cards and Talisman does not help with that at all. But at certain boards it is a good card, but you usually only want one.
That being said, with cost reducers Talisman can be used to gain 2 $5+ actions or treasures, which can be really nice. That could help to set up a bridge megaturn.