Hi all!
Well, I bought one at Petsmart about a month ago.
When I brought it home, Moxie thought it was a frisbee. I put it under a blanket on the couch, and she dug insanely until she uncovered it. Then she dug at it like an insane maniac, which is what she does to a right-side-up frisbee on the ground in an attempt to flip it.
I have a very old and small microwave, so it takes 9 minutes to heat the disk to maximum capacity. (Larger, more modern microwaves should heat it in about 4 to 5 minutes.) She has about 4 blankets in her crate which she likes to push around to create a monster nest (you need to see it to believe it). So before I leave I hide it on the bottom of her crate. She has never "unburied" it or chewed on it yet. I can't really tell if it makes a big difference for her or not. Her crate is large enough that she wouldn't have to lay on it if it's putting off too much heat, but so far her "nests" have been on top of it.
And I have to confess, I dig it out at night and use it for me!
I am constantly cold -- I heat it and keep it at my feet at night. It stays warm the entire night and keeps my feet warm!
So personally, I think it was worth the investment of $35 + tax. I use it for Moxie with unknown results, and I use it for myself with more than satisfactory results!
I really like that it does have a power cord. And the hot pink colour is fun!
FAQ 
If I put them under Syd's bed, they would be a big lump.
Lavander 
: Together in Singapore 
