Personally, I trust online storage providers for just one purpose: uploading things intended for the public. For example, if you want to attach a large file to a forum post (and said forum doesn’t allow files of that size), uploading the file to something like OneDrive or Mega makes sense, since that may be the only way to do so (and there’s not much point in worrying about privacy, since you’re sharing this file with the public anyway).
However, for private information, I would upload the data only if I have no other option (such a situation has yet to present itself), and only after applying my own encryption. I’d probably give the file some misleading name for good measure.
As for long-term backup, it’s usually quicker and cheaper to buy a hard drive than to use paid online storage. $0 online storage is nice, but you still can’t put too trust in any storage provider. Also, for only a few bucks, you can get a 5gb flash-based storage device that doesn’t require an internet connection, thus read/write performance will not be bottlenecked by slow internet connections. SDCards and USB flash drives are dirt-cheap nowadays.