While features such as the return of the Windows start menu and the Cortana personal assistant sound great, upgrading your Windows 7 or 8 machine to Windows 10 may not be the best idea at this time.
I have received a number of phone calls recently from people who performed the free Windows 10 upgrade and ran into all kinds of problems, including system freezes and lockups and in some cases Windows became completely non bootable.
Microsoft does give you a 30 day window of time to decide whether of not you want to keep Windows 10, and if not, you can roll back to the previous version of Windows you had. In many cases however the rollback feature can fail to run correctly, requiring a backup and clean re-installation of the original operating system and all programs.
In general I recommend waiting at least a good six months before jumping into a major new version of Windows, as a lot of major bugs and issues tend to surface early in its release which will be addressed over time with updates and Service Packs.