Having tried to move my data (bookmarks) from one service to another (Connotea to CiteULike and 2Collab), it should also be noted that even with backed up data, you can still easily lose a lot of time and effort and have to reduplicate work that you've already done. While many services claim quick and easy importing and exporting, the reality is much messier. Some things work, some don't. Some you have to do over again when moving. Just something else to factor into the equation.