Currently the best of the 3 Support items IMO, the 40 gold coins are amazing
Teammate clears a shoved wave? Walk there and get 200 free gold.
Got an Assist? 40 extra gold.
Compared with Spellthief's Edge, which only grants gold when you're sieging or fighting, Ancient Coin grants gold any time a minion dies, which is more or less the entirety of the game. As Janna and Soraka support I've gotten as much gold as my mid-laners by the end of the game with Ancient Coin, and have had a full build at level 17 with 4K+ gold left over (granted I had a really high KP that game). Statistically, Janna and Soraka have seen significant win-rate spikes since 7.13, and Ancient Coin Sona has been winning more than Spellthief Sona.
A bit overtuned? Maybe, but it's been fun.