Late last night, Microsoft began its advertising campaign for existing Windows 7 and Windows 8.1 users, promoting the upcoming Windows 10 launch on July 29. Exiting users are now receiving a notification allowing them to reserve their free Windows 10 upgrade, and doing so appears to have an advantage.
According to the Windows 10 FAQ page, reserving your Windows 10 upgrade will allow Microsoft to pre-load Windows 10 onto your device before July 29th, meaning when launch day arrives your PC will upgrade a lot faster as you don't have to wait for the upgrade to download.
When you reserve, you can confirm your device is compatible with Windows 10. Between reservation and when your upgrade is ready, the files you need for the upgrade will be downloaded to your PC to make the final installation go more quickly. Then, when your upgrade is ready after July 29, 2015, you get a notification that lets you get started with your upgrade.
This means that on launch day, everyone upgrading should have a super-smooth upgrade experience. The pre-load should take up 3-5GB of space, which may be an issue for devices with a mere 32GB of storage. Everyone else however, that shouldn't be a problem and you won't even notice.