I used it once. It seems to work well except that it greatly reduces the volume of the guitar. That's not a fatal flaw, just a minor annoyance.
I used a m-audio fast track using the ES, and had the same problem. I posted about this in the recording sub forum, it was suggested to use an external mic. Other places I have read that using a condenser mic is the way to go.
There are several good videos on YouTube on how to record an acoustic guitar