After hearing some good (and some bad) reviews of Windows 10, I decided to upgrade. First I tried my almost new HP desktop. Three attempts later, it put up a message saying "Reverting to your former operating system" - which was Windows 8, and the main reason I don't use that as my primary computer. I haven't gotten back to explore the issues.
Then, I updated my HP laptop, and here's my experience. (This may not apply to you - it's what happened with me.)