Consider its card type, any effect that affects when you could cast it (such as flash) and any other effects that could stop you from casting it (such as Meddling Mages effect) to determine if and when you can do this
This all probably makes it the least well designed card on this list, perhaps Stirrings takes that title
For that reason, I wouldnt be surprised if Sphere Grid dropped a bit in price once the novelty wears off, but it will likely always have a decent secondary market value because of the high likelihood for it to be synergistic with future Commander releases
then, each of your opponents can do the same
For the simpler pages, crayons and washable markers work great
Regigigas VSTAR exists as an all-or-nothing comeback tour for when the Shinx hits the Rotom-Fan, if you will, and we love it for that.