If you liked Gal Civ, you are in a similar situation with Elemental. Gal Civ II as a better game, out of the box, than Elemental for the most part, but it didn't get really fleshed out until it had a few expansions. Gal Civ II with all expansions is a much more complete experience than vanilla Gal Civ II. I expect that Elemental will continue to evolve in a similar fashion and will one day be a very complete package.
One of the key things will be to ensure that all of the gameplay changes and other changes don't add new bugs and major imbalance issues. Elemental won't gain steam with the gaming public if it does a '1 step forward, 1 step back' with each change. It is critical that the new changes be tested, both for bugs and gameplay. We don't need any more 'emergency' hotfixes for Elemental (unless I have missed hearing about some major bugs in 1.09e). We just need 1.1 to be stable and balanced, and then continue to build from there.
It is good enough to enjoy right now, and although I have had some enjoyment playing Civ V, I keep coming back to Elemental. Part of that is, as others have said, the huge potential in Elemental. Civ V has great potential too, and is already getting some mod love. I for one am more interested in playing and modding Elemental.
I would buy now if you want to support Stardock's efforts in making great games and if you are patient and interested in seeing the game evolve. I would say that is is 100% playable at this time, but it is not 100% complete at this time.