Only 1 experience about three years ago. The warranty was ordered through Amazon, but that should not make any difference. I had an issue with a shutter button on my 50d. I contacted Square Trade and they offered me the option to send it to them or to use a "local" shop that was authorized through them. The first local store was literally out of business. I called Square Trade and they gave me another name which is actually a store I know very well. When I went to that local shop (reputable by the way), they never heard of Square Trade and had no agreement that they know of with any such company (despite Square Trade actually giving me their name). This prompted a series of emails and calls to the warranty company. They finally said I could get the repair done (up to a certain amount) and they would pay for it. The shop felt the amount would not even cover costs. In the end, the final outcome was that they reimbursed me the amount they had promised and I made up the difference, since at that point I was reluctant to send my lens off to the unknown. Now, perhaps if I had sent the lens to Square Trade, this would have been less of a hassle. The actual amount of money I laid out was minimal and the final turnaround time was 48hour with the local guys once we figured this out. In the end, they did honor the warranty, of sorts, with a hassle.
FWIW, I also had an extended warranty experience with Mack. In that case, I sent them a lens that had stopped focusing after 4+ years of heavy use (EF-S 17-85) and it was back in my hands a couple of weeks later good as new. This lens issue was 6 years ago, however. Don't know how things are today.
Now, of note, I am getting emails from another warranty company (called, ironically CPS) about the 70-200 f/2.8 ii that arrives tomorrow. Never heard of these guys. They are half the price of everyone else. When you look online, there are lots of bad reviews for all these companies, but, the internet is full of bad reviews. Happy people usually don't post their experiences, so I never know what to make of this.
If I get a warranty on the new lens, I think I'll go with the Mack Diamond. I do take care of my stuff but you never know. Anyone else have any experiences, good or bad, with these companies?