Nahiri, the Harbinger was a card that I thought could be the future of Pyromancer's Goggles control leading up to Pro Tour Shadows Over Innistrad, though the new decks on the block diminished my faith in the Boros planeswalker. One copy of Nahiri did end up in the Top 8 of that event, though it was just one and it was out of a deck that only posted a 6-4 record in the swiss portion of the event. That said, Nahiri is starting to get looks for different reasons.
Jeff Hoogland, Joe Lossett, and Shaun McLaren have all been streaming with the card in Modern and/or Legacy. Matthew Tickal also posted this picture from his experiment with the card in a Valakut deck:
That Primeval Titan is about to find two copies of Valakut, the Molten Pinnacle before attacking and finding some Mountains to deal lethal damage. It only takes two turns of ticking up to ultimate Nahiri, and whether you're finding a Titan or an Emrakul, the Aeons Torn, winning the game is often enough elementary from that point in older formats.
Modern has plenty of ways to win the game at that pace, and Legacy most certainly does, though I wouldn't write Nahiri off in those formats just yet. A planeswalker that smooths out your draws and that can just win the game in a color pair with many of the format's best removal spells is definitely powerful.
Nahiri is already starting to tick up in value, though where this target becomes more interesting is with regard to the looming Eldritch Moon set. There's a lot of speculation that Emrakul is the blight on Innistrad, and a giant spaghetti monster to properly pair with Nahiri could lead to a very powerful and explosive Standard deck. At the current price, I think you can expect at least small gains from Nahiri, and there is real potential for the card to explode in Standard relevance and therefor price.