Google Drive, Dropbox, iCloud, and OneDrive are all available to try for free (with limited storage). So far I have used Dropbox and OneDrieve and like Dropbox since it handles better on the PC and my iPad. OneDrive is improving and I have used iCloud on my iPad mostly for pictures, there is a PC app available, so you can access your pictures and whatever else you put on it
What is your experience?
For starters, here is the free storage as of now:
Dropbox 2 GB
One Drive 15 GB
Google Drive 15 GB
iCloud 5 GB