First off, don't bother with my review, go grab the open beta and see for yourself. If you've played any Blizzard game recently, you've got battle.net installed on your machine and trying the open beta is as simple as clicking the Overwatch section and hitting "Install". It's about 6gb to download.
That said, here's my thoughts and impressions:
Someone at Blizzard wanted to replace Team Fortress 2, and they may have succeeded.
Not that Overwatch is a TF2 clone, far from it, but there's a lot of TF2 in the design of Overwatch, and a lot of things where it does what TF2 does but better.
Like TF2, Overwatch is a class based team shooter based around relatively short rounds. A round that takes more than ten minutes is long. Many games take less than five minutes from start to finish.
That sort of casual, drop in, drop out, sort of play is a delight in a world where many multiplayer games are designed around the idea of fierce competitive play for long periods and where dropping out after a short time is harshly criticized.
In addition to lightning fast games, the pace of the game itself is fast, a feeling helped by the fact that Overwatch is a much more vertical game than TF2 or most other FPS games. Several heroes have abilities that allow them to get above the fray and rain death down from a superior vantage point. Snipers definitely have an opportunity to shine in Overwatch.
Overwatch also draws strongly from MOBA's. The number of heroes and variety of powers far exceeds the usual found in an FPS and more are promised as development continues. I find this to be a welcome addition to the team FPS idea.
Powers are all on timers, except for the ultimates which require dealing damage or healing to charge. No ammo pickups, guns need to be reloaded periodically but all that takes is a few moments for the reload animation to play. The only truly limited resource in the game is health.
As in TF2, you can switch classes during a match, either after death or while in the spawn room. This allows for a more fluid play style than games that lock you into a class for the duration of play, and helps feed the feeling of Overwatch as a fast, flexible, game.
From a gameplay standpoint, Overwatch is excellent. Like all Blizzard products, it is polished, runs smoothly on even slightly older hardware, and looks good. The heroes aren't perfectly balanced yet, Bastion especially seems to be a bit OP when played right, but Blizz has a well deserved reputation for balance in multiplayer and while I'm sure that while they'll be more or less continuously tinkering with balance in Overwatch I'm also sure that they'll get it mostly right fairly quickly.
It's fast, varied, vertical, and altogether fun.
Overwatch is also a welcome change from so many FPS games in that it has female characters at all, and moreover only Widowmaker the sort of hypersexualized female character you so often find in games. Even better, there's a bit of variety in body types. Zarya is a beefy, muscular, woman and while she's the only one who isn't basically the thin waif type, it's unfortunately true that even a single non-waif woman in the gaming world is praiseworthy. And she's fun to play.
The hero lineup is still a bit of a sausage fest, out of 21 heroes, 8 are women, a bit more than a third. But that's at least a step in the right direction.
I'll admit, after less than three hours playing, I'm ready to buy it. I don't know if Overwatch will actually replace TF2 as my go to game when I don't know what else to play, but it could happen. And regardless, the time I spend playing Overwatch is time I'll be having fun, which is the whole point of gaming.