This is the best zombie experience out there. Easy 5 star. Yes, there could be improvements to areas, but compared to anything else it's a 5 star experience. I recommend playing on "hard mode". I never even tried easy mode though. I think people are too harsh in their criticisms. I love this game and can't wait for any DLC.
Suggestions: Go through the skill tree to plan out what skills you want later on; Use binoculars in high locations to find nightrunner hideouts / windmills and unlock them; Upgrade stuff at craftman such as molotovs, throwing starts, med kits, maybe UV bars; Always buy all the crafting components at craftmen; You're always getting new more powerful weapons, so buying weapons isnt needed.
There are dark and gritty parts which are great, but I also love being in the city with a vibrant blue sky and trees. After all, there will be nice days too during a zombie apocalypse. Running around at night is awesome. You gain bonus experience and chases can get pretty intense on higher levels.
The most problems I had were with co-op. If youre falling through the map, close the application. I know theyre working on a fix for this. I also played around a bit by trying to reach higher areas to fix this, but closing the application was biggest help. I never experienced this outside of co-op. I also played through the whole game during co-op. It can be a lot of fun playing solo cause I often get side tracked with side quests or other stuff as I play. Love this game