I have used my gaming computer with an unactivated Windows 10 (retail sellers are price gouging here, so nobody buys retail from a store in this country, if they have OEM on their PC's, that's it) for at least two years. Nothing except an "Activate Windows" watermark, and no customization options (which you can do some using registry), not even the kind of nagging Windows loves to do once in a while by showing a huge screen at every boot or something. It's definitely up to your ethics to buy it after using, but we recommend so.
Also, if you have a computer with an activated Windows 7/8/8.1, you can upgrade that PC to Windows 10 for free.