Ive have been using ZENFONE 2 for about 3 months now. Mine is ze551ml 4gb 32 gb version. So far, No issues at all. In fact, my zenfone 2 is getting even better with the recent updates. My batteryvlife is getting better, every day I got a little more than 5 hours screen on time with 13- 16 hours of usage, my pattern is web browsing, YouTube little biT OF lag free HD games for 40 minutes each day, playing music, mobile network connected but use wifi for browsing.
Call quality is decent. Signal reception is very good in city area, decent in suburban, but thats really depend on the service in your country. I live in Indonesia btw. maybe some people got problems with zenfone 2 because they dont know what to expect from a smartphone. Im talking with my zenfone two in its prime condition. So far, my experience is perfectwith zenfone 2.