This is a first impression of the game [If the 1.1 hours of game-time didn't make that obvious enough]

TL:DR
Rainbow Six in alpha with some nice little touches, but nothing worth overlooking all its issues.

Zero Hour is a tactical 5 v 5 shooter. One team defends a hostage with a bomb strapped to him, the other tries to either kill the opposing team or defuse the bomb (note: I have never once had a situation where the bomb has been defused)

The gunplay is... interesting. The devs have tried to make it as realistic as they can manage. The problem is, they made the recoil be either uncontrollable or stupidly hard to hit anything after your third shot. This is best exemplified by the Uzi, the only primary weapon available for the hostage-takers.

The weapon variety isn't great either, but quality over quantity is one of the main principles in the game design here. There are 4 primary weapons in total, one of them being a ballistic shield.

The tactical side of the game, which is supposed to be the main draw of it, is just stomped into pieces by the community. Communication seems to not be a priority for most players. Others communicate too much. The worst of the crop are the ones that scream uncoherent bablings.

The maps are well-designed from what I can tell, though I'm not experienced enough to tell.

To conclude, buy this if it goes for 50% off or something of that sort, but I wouldn't recommend paying anything more than 5 euros for Zero Hour in its current state.

P.S. I realise this is an early access game, but that doesn't mean I shouldn't leave my thoughts on it. I wish the developers and community the best of luck. In a couple of months/years this might be something truly spectacular and a pillar for the tactical shooter genre, but right now, it's not even worth looking at.