I have been using Microsoft Windows 10 for over two month now. Cortana is a big help and right now I am more or less using it as my "Search" cum "Run-as" feature. The side notification center situated on the task bar is a big help. Quick look up and turning features and settings of my machine on-off. The outlook and calender apps are the most used one at the moment as I could not find much exciting otherwise there. The good news Microsoft will update this to the RTM version of Windows 10 for free. Anyways with VS2014 community edition now also free its good for developers.
One great thing about the Windows 10 preview built is the Windows Insider Hub app, through which you can give feedback and suggestions directly to the Microsoft team, which is really really cool. I can up vote on features requested by others which they would like to see. There are different categories under which you can directly provide your views and must haves directly to the concerned teams.