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Dropbox syncs pretty fast, and you can pause it at any time if you are on low bandwidth + you can access your files on the website. I save all my work in it and don't ask myself again if I have made a backup. It works well in all cases, in my experience.
OneDrive (Microsoft's equivalent) can be slower to sync and is awkward on a Mac, but otherwise is well integrated in Windows 8 and allows you to edit files using Office Web applications (lightweight versions of Word and Excel etc.). You can now pause syncing but it is still often slow in my experience. But it offers 15GB by default which is better than Dropbox's 2GB. If you decide to take a subscription, then Dropbox is great for professionals (several TB of space) and OneDrive better for individuals (1TB if you buy Office 365).
Haven't tried much Google Drive, if you already use Google Docs a lot then it's worth it.