So far I've managed to play 3 games with Young Nick so far involving this card. Unfortunately my testing is limited by the sets I own.. Fool's Gold appeared in 2/3 games and Witch appeared in 2/3 games.
Young Nick used Astrolabe game 1 to assist his Witch/Fool's Gold/Chapel strategy, but lost to my deck that ignored Astrolabe (using the same key cards). I think Astrolabe may not have been a correct purchase that game, and that even if it was, it didn't change the game so much that it makes me say, "hey this is why this card should exist".
Game 3 I did feel like "hey this is why this card should exist", but man it had a lot of support. Fool's Gold, Workshop, Smithy, Tactician, Ghost Ship, Merchant Ship, Pearl Diver, Duchess, Haven are what I remember of the kingdom. And spy and Astrolabe. I purchased an Astrolabe to go with Haven and Tactician to make it incredibly likely that it was used to purchase Victory cards. It was used for Province/Duchy.
After playing these games I want to test the card at 2$ with Gardens/Silk Road to see if they form an unstoppable rush or not. I think it may very well stay manageable.
Witch made it a lot harder to care about Astrolabe, I feel, and I'd like to give Astrolabe a reaction from hand so that it can block curses and be used with gainers. The primary reason it doesn't have such a reaction is because the original contest prohibitted that, yet at this point I feel like it is best to do what is best for the card.