I haven't played DayZ, but I am a huge fan of the videos on YouTube. I didn't want to purchase Arma2 and Op Arrowhead, just to play a mod. But... I almost did on a few occasions, even had the purchase page in Steam open.
From what I understand, WarZ runs the same engine as War Inc., according to developer commentary on a gameplay video. However, it has been tweaked heavily for WarZ. Its even made by the same company, well a subsidairy of the parent company, but many studios do this. Also, I believe they tried to keep things hush hush until the annoucement of the stand alone DayZ game (Just my opinion). Even the developers on gameplay commentary aren't shy about stating that the game plays similar to DayZ.
Anyhow, I managed to cough up some dough to play this game. I will do the same when DayZ stand alone comes out. I actually think both games will benifit from this "competition". Huge zombie fan here. Anyone else catch The Walking Dead Last night? They have gotten to my favorite part in the comic.... Well it follows it somewhat, lol.