Many have evaluated online backup systems on the web. Query Google to find comments of all kinds.
Search google for online backup reviews 'Mozy.com reviews', Iron Mountain reviews, Carbonite Reviews, etc. Then look up positive terms like 'Mozy is the best', 'Carbonite is great'. Then try negative terms like 'Mozy restore problem', 'Idrive problems' or see why customers 'cancelled Mozy' or 'cancelled Ironmountain or other companies.
ALWAYS browse their legal Terms of Use and Privacy Policy! There could be hidden fees for data overuse, or they describe themselves as storage on an 'As Is' basis and more.
For example, Ironmountain charges more if you go over certain amounts of MB of your account. Look to these hidden fees or if the policies are subject to change during use of their service. Some policies say that rates can be increased for quarterly use or more.