Nissa lost a lot of value when it both say a reprint in a duel deck and the Standard deck it was played in fell out of favor with rotation. This week though, on the eve of the the Aether Revolt release, Nissa has about doubled in value.
With Oath of Ajani and Sram's Expertise becoming Standard legal, people are expecting tokens to make a comeback. There's even Walking Balista to flip your Archangel Avacyn // Avacyn, the Purifier's on command! Dromoka's Command rotating was a big hit to the deck, though the deck still has a lot of power, and the banning of Smuggler's Copter definitely makes casting planeswalkers a lot more inviting.
This is one to watch in the early weeks of the new format, as Oath of the Gatewatch will be rotating out with the release of Amonkhet. Either the deck will be successful and there will be a few weeks to sell into the hype, or the card will come crashing hard in the short term. I definitely wouldn't buy in with the current price point, and would probably sell any copies I wasn't planning on playing as both the current price and the risk of holding are high.