`rsync-for-android` is actually pretty good if rsync is what you actually want; android-only of course, and FolderSync has a slightly nicer UI-wrapper (using sftp directly, rather than actually rsync'ing) and "experimental" auto-push-on-change, so between the two of them my phone and tablet now "play well" with my existing linux-based photo workflow. (I agree dropbox has many advantages, and a simplicity that comes from not having features... but with rsync, I can keep "just the last 30 days" of my full photo archive on my tablet, nothing else gets me that particular kind of control.)