I thought to throw in SugarSnc since it hasn't been mentioned. I also have Dropbox, Drive, Skydrive but Sugar is the one that seems to tick the most boxes for me. It is very easy to choose what folders you sync and to which devices as different devices font need all files. You can add passwords to public folders which is a nice touch. And pricing (at least when I compared 18 months back) was quite competitive.
...not really. On Android I do find it burning CPU/battery quite often so I updated settings to only sync when plugged in and on WiFi. It is a minor limitation for me and can do a manual sync if happen to need something from my PC and am on the move.
My biggest bug bear is it asks to upload my photos every frickn time I log in despite having deselected in settings and pressing the 'never' option.
