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I have never used a third party insurance for any iPhone I have ever owned. This is partly because I have used my credit card to purchase my phones and my CC provider covers me for damage or loss for 2 years. After that the phone is not valuable enough to me to continue paying for coverage. I do purchase Applecare as well but have never had the need to use it. But it is nice to know I have technical support for 2 years if I need it. I am careful to never leave my phone alone, plus I am very careful with my phone.
The one thing Applecare does not cover is loss or theft. So I think this would be the biggest thing to consider in regards to third party insurance. You would have to read all the fine print for any third party insurance to make sure it will fully cover loss or theft and what, if any, kind of deductible there may be associated with it.