May be a silly question but where is all the data that is synched stored? If it is not resident on your computer over which you have control is it not a HUGE potential security breach to store passwords elsewhere?
Not a silly question at all. As with services like Yodlee, the passwords are stored on central servers outside of your direct control, so you have to trust the company you send these to. You'll have to determine for yourself whether the benefits of xmarks or ffsync outweigh the potential security concerns.