Dropbox strives for 100% uptime and availability, but it is unrealistic to guarantee that. Rest assured knowing that even if the Dropbox service is down, you can still access your files from the Dropbox folder on your computer.
In other words, any Dropbox service downtime only affects syncing and online access. Dropbox copies files stored on your hard drive when it syncs, so your files will always be accessible from your hard drive even when Dropbox is down or when you are offline.
If you would like to receive Dropbox service announcements by email ahead of any scheduled downtime, you can opt in from the account settings page. Additionally, we always keep users updated about possible bouts of downtime through the Dropbox forums and our Twitter feed.