This offer expires 29.7 - 2016 after which you will have to pay for the upgrade.
Windows 10 has had its fair share of trouble so this expiring "offer" is not regarded all that positive by all. Windows 10 goes to the cloud and it will stay there. This might reduce the control you have over your computer compared to prior Windows installs.
Microsoft might hope for a "hurry and upgrade before it is to late" effect. IMO you will do better with Windows 7.
The price for an upgrade will be about $100 after 29.7