As with most things, it's a judgement call, though my own judgement has been that it's rarely worth it. In the UK at least, the manufacturer's warranty is not all that covers the goods, so the prospect that it may fail the day after the manufacturer's warranty expires is not in itself reason enough to buy an extended warranty. Whilst it's not precise, in UK law goods have to last a reasonable amount of time, and one is still entitled to redress even outside the manufacturer's warranty period, which is why warranties in the UK carry a notice advising the buyer that the warranty is in addition to, and does not affect, your rights under consumer legislation. If the extended warranty is a small enough proportion of the price of an item that you expect to have a long service life -- for example a washing machine as opposed to a mobile phone (to choose non-photographic examples!) -- then you may think it worth it for the peace of mind.
I took out a similar sort of policy for household appliances, through our energy supplier at the time. They kept pressing us to 'upgrade' the policy every so often. Eventually, we were paying them GBP 650 a year, more than our house buildings and contents and photographic insurance combined. For reasons that are too off-topic for here (they were unable to identify and repair a fault that persisted for three years), we cancelled the policy, and received the rudest and most impertinent telephone treatment we've ever had from any service organisation.