I’d be inclined to(unless your specific selection of hardware and software makes this an unacceptable burden) at least ‘windows10-ize’ any Win7 or Win8 systems that are entitled to the free upgrade; but at least for Win7, I’d then roll them back. Doing the update, at least in my limited testing, makes MS accept the license key/BIOS-embedded entitlement as valid for Win10 clean installs thereafter(while merely trying to do a clean install, using a key that was entitled to a free upgrade, doesn’t work).
That keeps your options open for when Win7/Win8 support runs out and it’s either Win10 or the suicidal folly of running without security updates. I’d use drive cloning, rather than trusting the built-in rollback feature; but this is a cheap way to hedge your bets on any licenses you currently own.
Now, as for actually using it; I’d tread more carefully. At very least, no way in hell would I tie my local login to a microsoft account, nor would I ever allow a bitlocker key to be ‘backed up’ for me. Cortana can stay in Halo, where she belongs, and so on. All the privacy-related checkboxes would have to get checked; and then I’d remain wary(Arstechnica has a good piece). I await further testing of this sort of thing; and am going to be doing some myself when I have time.
That said, some of the vague-and-sweeping TOS is not specifically tied to Win10; but to the various ‘cloud’ stuff integrated into it; and there you are unlikely to be comforted by any vendor except the ones that specifically cater to privacy enthusiasts, discuss their (non) knowledge of your encryption keys in great detail, etc.
“OneDrive”, “Cortana”, “Bing”, “Outlook.com”, etc. are all going to be utter privacy screwjobs; but I’d be very, very, surprised if you’d like the TOS for the equivalent cloud-storage, digital-assistant-AI-personality-thing, search engine, and free webmail, from Apple, Google, or most others. ‘Cloud’ TOSes are just all kinds of hostile; and while Win10 brings greater default integration with MS’ online properties, so long as you can avoid those they aren’t intrinsic to the OS; nor are the equivalents built into other OSes going to be any nicer(Siri probably can’t keep a secret; ‘OK Google’ likewise, Gmail very unlikely, and so on.) The same goes for online tracking for ad-network purposes. MS is an active player here; but that applies no matter what OS or browser you are using; and the best you can do is follow the usual defensive advice; but even if you are using Plan9 and Lynx you’d still be potentially at risk.
As for Apple, I’d be wary: I do adore how their threat to implement device encryption that actually works is turning the FBI flack’s face a funny shade of purple; but they are actually even more aggressive than MS when it comes to tying everything to your itunes account(which is usually also tied to a credit card, so you are either doing some fraud or they have a very good idea exactly who you are). They also do a hard-sell to get you to link your local login to the cloud, they have an even more aggressive ‘app store’ adoption(you can still disable the prohibition on unblessed software on OSX, at least for now; but OS updates and the ‘included’ iLife/iWork stuff are app-store only; and they’ve been more successful than MS at shifting 3rd party devs over to the app store).
Google, of course, has relatively few offerings outside of mobile; but is also not too much to be trusted.
Long story short-ish; I’d update your keys, just to keep the option; and see what the reports are about how chatty Win10 is when told to shut up.
If I were to go Win10; I’d entirely avoid contact with any MS ‘cloud’ services. No OneDrive, no Cortana, no bing-search-from-start-menu, no connection between local and MS account, ideally no app store. At that point it might be safe.
If you do want any of those features, though, the news is bad: yes, MS appears to be crazy evil; but you are unlikely to get better terms from Apple or Google or Amazon or Dropbox, on ‘cloud’ stuff. Unless it’s one of the very-specifically-avoids-ever-touching-your-keys-and-only-holds-the-encrypted-blobs vendors; it’s an open book(even if the TOS doesn’t say it, the 3rd-party exception to the 4th amendment is a gaping weakness that is so trivial you must assume it to be routinely exploited).
