This is a really good burn spell. For X and double Red, it deals X damage divided as you choose among any number of target creatures and/or players. This makes it better than old Fireball because you don't have to pay additional mana for each additional target. It's sorcery speed, but it can help Red players find those last few points of damage.
People are super excited about this being in Modern, but I would like to point out that it's not quite as good as the under-appreciated mythic rare Aurelia's Fury from Gatecrash. That card is an instant and costs XRW, and can tap creatures and stop a player from casting noncreature spells... why doesn't it see play in Modern. but forget they have a superior option already in Fury. But I digress.
Despite being outclassed in Modern, I'm sure plenty of Standard players will be casting it as the format is losing Stoke the Flames and Lightning Strike (barring reprints). So it's time to dig up some old beat up Tempest copies of Rolling Thunder and finish off some opponents!