Yes, this means that you pay 0 for X, which allows you to play it for free. That means it will just be a useless artifact just sitting there, right? Actually, there are ways to place a charge counter on it.
In Commander (aka EDH), there are a few cards that interact well with this card. For example, there is an artifact called Coretapper that can tap itself to add a charge counter to target artifact. There’s a similar one called Power Conduit. Also, Energy Chamber does the same thing at the beginning of every upkeep. There’s also Surge Node, but it is limited in how many times you can add counters, since the Node only starts with six. You also have to pay mana to move the charge counter. Since there are so many options to put charge counters on the Cornucopia so easily, it’s actually a pretty nifty mana rock. You can play it for free on turn one and then have charge counters build up on it over time. Out of nowhere, it can offer you a lot of mana.
On its own, without the proper charge counter support, it’s pretty underwhelming. When X is set to 1, Astral Cornucopia is basically a Manalith, giving you one of any color mana for a tap. It’s not as good as Darksteel Ingot, because that’s indestructible. So what’s good about this? It obviously gets a lot better when you invest more mana into it. First of all, having extra mana to sink into things definitely happens in Commander. So the possibility of being able to sink 6, 9, or even 12 mana into this is certainly there. You could have it tap for 2, 3, or 4 mana per turn quite easily.
Then, there are cards that can double the number of counters on something. These include Vorel of the Hull-Clade, Doubling Season, and Primal Vigor. Vorel, in particular, is a Commander that makes very good use of this card. There are also cards that have the Proliferate mechanic to add more of a given counter on a permanent. These include Contagion Engine and Tezzeret’s Gambit.
The Cornucopia was never a high pick in Limited when Born of the Gods was in print. It was a bit too mana-intensive to be usable in Constructed, either. There was speculation early after its release that it could find a home in such Tron deck in Modern. There are better cards in that sort of deck to invest mana into, however. It’s become a Commander-only card. Being Commander only is fine, as it’s only a rare. Foils of this card are still sought after for their casual utility.
There are enough silly things you can do with charge counters, such as with Lux Cannon, that this definitely has a home in a number of decks. Commanders such as Vorel, Rosheen Meanderer (who can tap to put at least 1 counter onto this with her ability), and Kurkesh, Onakke Ancient (Artifact shenanigans) are happy to have this in their 99.