Generally you should only buy insurance if you would not be happy insuring yourself (e.g. buying a new one when it breaks). I fyou buy insurance then the profit margin and overheads of the insurance company are an extra cost over insuring yourself.
Buying insurance works best for expensive rare occurances (e.g. major car crashes) and not so well for cheaper, or more common problems (e.g. broken consumer electricals).
This is especially true with product insurance on products with a short shelf life (e.g. the latest iPhone) as their value drops quickly and better versions are introduced.