I'm a big SugarSync fan and find it pretty easy to use.
It has a starting amount of 5GB, so initially it has an advantage over Dropbox (can be made up by referrals, but that takes time). I'm now up to 8.5GB with referrals.
I'm slowly moving all my books into SugarSync as I prefer to have them in a readable state first (my library has a huge number of different formats that need converting and tidying up).
I'm also tempted to purchase some online storage, but 100GB or so is not sufficient and I would Ideally like to have around 1TB available. I unfortunatly don't know any one that supplies that amount for a reasonable price.