I've done a little testing with it in the past, and frankly, had amazing results.
Definitely more refunds than normal, but I sold a boatload on launch day.
So here's my thought...
There's got to be a way to do blind copy where the buyer is still satisfied with his purchase.
Obviously you've got to have a kickass product, but there's more to it than that.
I've got some ideas, but I'd like to hear what the other writers in here have to say first.
(Actually, here's one: Make sure the customer knows when reading the letter that your idea isn't "new or shiny" even if it is - in non-blind copy this has actually worked very well for me)