I've read some disturbing things about Dropbox. I use it for innocuous stuff, but apparently the files are not encrypted and Dropbox employees are free to troll around in them. They tell you straight out that they will only store one copy of a file even if multiple "customers" save it, so they are looking at everything you store.
Obviously on a PC, you can encrypt things in ZIP files or other files, but it's more difficult on a tablet. If you're clever though, you can use multiple online services to store quite a lot of data for free.