Because I used Windows Live Mesh in the past (or maybe because I tested out Skydrive in the past, I don’t know), I was eligible for the free upgrade. Without this free upgrade I probably wouldn’t have used either Google Drive or SkyDrive because their free storage options (5GB and 7GB respectively) were not quite large enough to contain all the contents of my pictures (7.5GB) and key files (2.5GB) directories.
While I was an enthusiastic early user of Dropbox,
I prefer SkyDrive over the new Google Drive and DropBox primarily because I can sync any number of disparate existing folders from my PC. Google Drive and Dropbox limit your sharing to only one dedicated folder.
[Update: After reading the Paul Thurrott piece listed below, I discovered that, unlike Live Mesh before it, Sky Drive does not yet allow you to sync multiple disparate folders . It appears to have the same dedicated folder limitation that Google Drive and Dropbox have. Alas, while I’m happy to have the 25GB upgrade, it won’t be used until I can use SkyDrive to sync multiple folders.]