I have had it on my Clio since I got it. Lets just say that I have had my moneys worth out of them over the 4 years of ownership
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The only drawback is that the repairs have to be carried out by a VAT registered garage which means when I have had to use it I can't go to my regular place and VAT registered places tend to charge more per hour in my experience. For the most part this isn't much of an issue as most bills wont come to the £500 limit but if it's a possible big one then you will need to take it into account.
Also as above you need to have something go wrong to be able to call them out. (I ended up driving mine for a while with a faulty coilpack until the light came on) then I was straight on the phone lol.
When all said and done the AA from everything I have heard are probably the most likely warranty company to pay up rather than trying to wriggle out of it as well as covering more parts than most.
Even though I've made about 5 claims in the 4 years I don't think I've ever had to pay more than the £110 or so I had to pay this year for it. They try and put it up a bit if you make a few claims but I just ring up to cancel it at the end saying I'm to skint and they work something out for you lol.