Dropbox is very secure. I use it just to sync my data which keeps on changing. I keep this data on a separate partition. Dropbox gives you 2GB free and you can earn up to 8GB free for introducing people. Thus, a total of 10GB free. So far I have 7.5GB free.
I have no problem syncing anything so far, even Microsoft Outlook data file. The best thing about Dropbox that it syncs only the changes made in the Outlook data file rather than the complete data file. For example, if I receive two new email and I delete two contacts, it will sync only these changes, while Outlook data file is closed. Most people pay about $60 to buy Outlook syncing program and they have to do it manually.
If someone introduce you to Dropbox, they get .25GB free and you get .25GB free too. So, you start at 2.25GB free rather than 2.00GB.