The best part of this split card is Expansion. It's absurd with how many situations it can deal with. Explosion can also be good, but you will play this mostly for Expansion. Here's a couple of Modern decks that would like to have it in their main lists.
Expansion is perfect for any blue-red Storm deck both as an accelerator and as a counterspell. But it's better than counterspell, which simply annuls the target spell, while Expansion copies its effect with a new target.
Jeskai decks are some of the strongest in Modern, and with the help of Expansion it can now deal with a few bad match-ups as well. Here you can also use the Explosion side as an alternative removal and a card draw engine.