I was considering upgrading as soon as possible from Windows 8.1 (for gaming only, usually I am using Fedora for work :) ), but some article about Windows 10's new privacy terms made me think twice :
http://www.numerama.com/magazine/33357-windows-10-microsoft-et-vos-donnees-privees-ce-que-vous-devez-savoir.html It's in French, sorry, and I haven't the time to fully translate now. But it contains concerning things about what kind of information is sent to Microsoft's servers. Here are some points from that article :
For example if you log into Windows with a Microsoft account, some data and parameters such as web browser history, bookmarks, and even wi-fi passwords (?!) will be collected for synchronization (can be disabled).
For each user of a device, Windows creates a unique ID for advertisements. This ID can be used by application developers and ad networks to offer users more relevant advertisements.
Furthermore you should not use BitLocker, as your recovery key is automatically saved online in your OneDrive account, and can be disclosed in case of legal actions (of course :) ). According to the article, other network data and what applications you use can be transmitted to Microsoft, and not all of these can be disabled.
Edge (the new browser, successor to IE 11) sends to Bing any information entered in the address bar when AutoSearch and Search Suggestions are enabled, even if you select another search provider by default.
Also, not surprisingly I guess, a lot of data is collected due to Cortana.
Collected personal data can be stored and processed in the United States or in any other country where Microsoft, its affiliates or service providers are operating.