I haven't played another FPS besides Overwatch in months, which makes me qualified to state that Overwatch is the best FPS on the market. That said, until today, it was lacking a deathmatch mode. And players wanted deathmatch. Though after playing a few rounds, it's clear why adding this mode was not a priority.
Before this update, the only way to play the game was through objective-based modes. Players capture a point or a flag or something. But you were always capturing, which lacked a certain, "I just want to kill something" quality.
I played the new deathmatch mode for a few rounds while writing this post. It's obvious why Blizzard took long to add the mode: Only a few of the characters work in deathmatch.
The beauty of Overwatch is twofold. One, the maps are fantastic with open layouts and slightly interactive environments. But moreover, the characters are crafted in such a way that most have limited use outside of a team. That's fine. It works in Overwatch as the game is inherently collaborative. Since the game was designed around capturing points, a chunk of the game's characters are designed for defending, holding or healing -- all which are irrelevant in a free-for-all.
So far it looks like Hanzo, Roadhog, McCree and Soldier:76 are the most used characters in deathmatch. Others work, too. Mei can cause straight-up pandemonium by freezing everything that moves, but without the support of someone else to kill her prey, she's not exactly right for deathmatch. Sombra plays well too though her hacking ability is downplayed without team members. Others like Genji can be devastating in the right hands.
Still, out of the 25 characters in the game, most are worthless in deathmatch.
The mode still has a few oddities to work through. Assists are displayed as kills except on the scoreboard and occasionally the spawn points are too close to the action. Those are things I would expect Blizzard would fix in an early update.
Overwatch players have been calling for this since the beginning for a good reason. Deathmatch has long been a staple in FPS and even if the mode doesn't completely work, a game like Overwatch feels incomplete without it.