I tried Windows 10 the first day of the rollout (today!)
Installing Windows 10 on my 5 month old HP Stream 11 was easy.I have no comments on that process.
Visually two things stand out: windows are all white except for a very thin aqua blue margin and my slow laptop seems to run the UI faster. I don't know how much of the speed bump is making the code more efficient and how much is doing away with some animation effects.
The desktop now seems like a mixture between Windows 7 and 8.1. The start menu is back and the bottom icon navigation bar is always visible along the bottom of the screen unless you put an application in full screen mode. Clicking the windows start menu in the lower left corner of the screen brings up a combo: classic looking menu on the left and the metro large icon interface like Windows 8.1 on the right side of the popup - but this popup only covers about 30% of the screen. It all seems a bit odd to me but I really like it - after using it for ten minutes (it took a little while to adjust to the new interface). I think the new interface in general is very well done. I haven't spent enough time with the new Edge web browser to have a firm opinion yet, but it seems functional and the reading view does a good job of reformatting web pages without advertisements for comfortable reading.
Windows 10 Cortana, similar to Google Now and Apple Siri, is always available just to the right of the Windows start button. Just like the search utility on Windows 8.1 this is the way to quickly find stuff in the system. Need to change the PATH, then just type 'environment variables' and instantly the environment edit utility is shown. I think this actually works a little better than Spotlight on OS X and quite a bit better than hot key search on Ubuntu. I tried using Cortana for searching for things in my community. It did OK, but will hopefully get better as it gets access to more of my life context.
I will spend time locking down some of the privacy settings. I alreaded deleted the Skype application because it leaks a little too much personal information for my taste. Cortana is configuraable for setting which types of information are collected. It pays to take the time checking possible privacy settings for products and services from Microsoft, Google, Facebook, Apple, etc. I tend to keep my Linux laptops more locked down, privacy wise, than my Windows and Apple laptops. I try to strike a balance between having some privacy and also enjoy available products and services.