I've had decent results and its what blasted my business out of a cannon when I first started. This last round of EDDM didn't work so well. Note to self: Don't sent out EDDM to an area that the homes catch fire and burn down.
I actually searched the site for EDDM after I posted this and decided it's not for me. I believe it was Mikey's comment, "how often does a postcard prompt you to buy " (paraphrasing) that did it.
My answer was never.
Its good if you are just starting out and have no leads. Once you build your business to a self sustaining point, you dont really need it. This is why the Gurus feed off the noobs with their EDDM magic nonsense.
We have used them when we open new drop off locations. It's a good way to saturate areas close to those locations letting people know we are now in the neighborhood.
I think the mistake that people make when using EDDM is that they are using it as a sales tool rather then a brand builder. As others have said, you don't really get a lot of sales from them, but it does help with brand recognition.