Short review for Windows 8.1
Installed last night on my 4-year-old laptop, running smooth without much troubles in getting the drivers. (except for Nvidia I need OEM driver).
Improved a lot:
-Default home screen now makes much more sense, no messing up by installation of new desktop applications,
can be grouped, more tile size
-Can boot to desktop
-Notification now works very well, logging with my Microsoft account, it automatically logon Skype, Mail, Calendar and more.
-Boot and shutdown even faster than Windows 8
-Lock screen notification works well, allowing me to see unreal mail counts or Skype message counts when screen is locked.
Calendar can be sync with Android default calendar, and two-way meaning that you can make change on Outlook Calendar on both sides
*PS: Office 365 works well on Windows 8.1, no complaints so far.