It depends what sites you are using. Some will be very secure, others not so much. Just like you would check to see a site is using SSL before entering your credit card details, apply the same caution to selecting a cloud site to keep your data. There are plenty of providers to choose from, and you can pick more than 1 (eg flikr for photos, amazon for music and Google docs for documents). One of the benefits of using the cloud is it makes it easier to share your stuff with others, but check that the sites you use have the right level of security and privacy controls for what you want before uploading valuable files. It is important to take the time to learn how these work too, so that you don't inadvertently make sensitive files public.*Everything is encrypted and available by sign in only.
Make sure you have a cryptic password and be sure to change it once in a while.