I'd shy away from purchasing anything from an UNauthorized dealer. There are plenty of authorized dealers that will come off of retail prices enough to make it worth buying from them and getting the warranty is worth a lot.
my 2cents.
If it was a $150 purchase, maybe I can see risking it, but for more than that... I like having the factory recognized warranty!
I haven't had any experience with purchasing a warranty from an unauthorized dealer, but I've heard them describe them to me. I forget all the details they told me, but for what it's worth, it wasn't enough of a savings and I still ended up going with a Denon authorized dealer to get my equipment.