Well, that's not completely fair. The extra 20GB of storage only lasts for one year, so when those twelve months are up, you revert back to the default 7GB of storage unless you pony up for the upgrade. Even so, having one year of extra storage is a nice perk, especially for existing users. Contrast this to the free Dropbox storage you might get with a Samsung Android device, but only when you buy a new smartphone or tablet. This Microsoft offer is being extended to both new and existing users.
Since I have a Nokia Lumia 521, I got that email over the weekend and proceeded to go through the steps to activate my extra Skydrive space. It was as simple as clicking on the link in the email and hitting the confirm button on the resulting page. If you got the email too, make sure you add that storage before the offer expires at the end of January.
If the extra 20 gigs isn't enough, you could always invest in something like the WD My Cloud to augment that storage. Then you can get terabytes of space, though in a different kind of package.