I've just resurrected another cloud storage space... I had previously been using iDrive Backup, which is an automated backup service: not for syncing or sharing, it just would back up your files which you could restore if something awful happened. I used to back up my contacts, serialnumber/logins database, keychain (I use a mac) and a few precious photographs which were irreplaceable. I stopped using it when I decided to clone my drive, and use Time Machine.
Syncing, though, thats a whole other ballgame. Dropbox was first and in many ways is best for some things. I went for Sugarsync next and I find that VERY useful in terms of sharing... I often get Virtualbox appliances via Sugarsync, much better than downloading... I also have had a couple of others... skydrive... heaps of space but pretty ineffective as anything more than storage... Google drive... same... Box.net is ok, I went for it only for the free 50GB last year but in truth havent used it much.
idrive now, though... its doing sync, and its being very generous with its starting point. You start with 10GB and get extra when you install the desktop client and the mobile client, follow on twitter, and "like" on facebook (500Mb per action there). You get 1GB per friend who signs up from your link... but so does the friend.
I'm uploading my calibre library as we speak, mostly to test speed but also to see how it behaves when accessing via the mobile app, or safari/chrome, in terms of getting books into your reader of choice.
You can get up to 30GB total for free, via referrals and the suggested activities. I think it may even replace dropbox for my books storage/sync, because its a lot more available space. I dont have many books now, but who knows what madness will strike me and start me off getting more and more... I'll edit this post when the suitability of Calibre lib sync is tested, and I'll edit my sig to include the referral link which you may, or not, choose to use, to get your first 11GB.
 And its back to Dropbox or Sugarsync for syncing the Calibre library. iDrive did the same as box. Synced the same stuff over and over, as if the first sync had never occurred. And it was as slow as a wet week. It would be good though just for basic syncs. Large ebook libraries, or huge numbers of photos... I don't think so, really.