VERY linear.
The zombies are just as fast as you and don't change direction that quickly, so it's quite easy to avoid them. That, added to the fact that you can one-hit everything (and with the size of the zombies, you almost never miss), makes you practically invincible.
There are so many things that could be easily added in later levels to make this much more enjoyable game. Different kinds of zombies (e.g. faster ones, exploding when shot, more durable ones,...) more weapons (weaker longrange (rifle), stronger shortrange (flamethrower, shotgun), something that pierces through everything in its way,...), obstacles, etc.
But with a working, fluent mechanics like this, it would be almost a shame not to make it into a larger game - e.g. something with a story - for example an adventure in a zombie infested city, survival horror, I don't know... I would gladly give five star rating to any more complex game based on this mechanic, because now it is just too linear and becomes boring quite quickly. The competitive, score aspect isn't that fun, as the final number depends on everyone's patience more than on their skill.