Overwatch is currently free to play all the way through until the 4th of January, and if you decide to pick up the game, its also heavily discounted right now.
This isn't the first free to play window the game has had, but they usually only last for the weekend - this time, you've got two full weeks to give the game a thorough road test. You'll be able to play the full game during this time window, and any progress you made will, of course, be kept if you decide to buy.
If you pick it up, then the Legendary Edition is currently half-price at just $20 - with the standard edition at just $15. The legendary edition contains many extra skins and cosmetics for various other Blizzard titles such as World of Warcraft and StarCraft 2, so if that's something you care about, could be the better value.
Blizzard has other games discounted right now, with cut down prices on Diablo 3, StarCraft, WoW, and Hearthstone card packs through its battle.net launcher.