There have been other enchantments with a similar effect, but with drawbacks. These include Winter’s Night from Alliances and Overabundance from Invasion. Zhur-Taa Ancient, a creature from Dragon’s Maze, also happens to have that effect, and it’s a 7/5 for 3RG.
So does giving this card Flash suddenly make it a bunch better? Interestingly enough, this card works best with Kruphix, God of Horizons, and not Karametra, God of Harvests. Since the effect works for all players, Kruphix’s ability to turn any excess mana into colorless mana is really good. Five mana is a bit much for an enchantment like this, though. It is more stomach-able if you’re playing a Kruphix deck in which you’re using leftover colorless mana and only paying 2 Green for it. That’s something I’m sure R&D took into consideration in costing this card, as Heartbeat of Spring with Flash could've proven to be pretty powerful.
While it's a bit expensive to cast in Standard, it's going to see some Commander play, certainly. It's especially good alongside Kruphix, for a bit of extra redundancy along with the cards mentioned earlier. It's important to note that this effect does stack upon multiple instances. It’s definitely a usable enchantment, and using the Flash to cast it at instant speed can help you take advantage of it first.