Hi Paul, I emailed Bintel about the 16mm edge on eyepiece. Don from Bintel stated that he had tried one of these eyepieces terrestrially in an ED80 scope and that the edge of field was crisp. He also stated, however that he has yet to try one under a dark sky. Here is a cloudy Nights review of the Smart Astronomy EF eyepieces which appear to be the same as the Orion Edge On's.
http://www.cloudynights.com/item.php?item_id=1684
Having only five elements should make these eyepieces produce sharp, bright and well contrasted images on -axis though i would imagine the edge of field will be astigmatic to some degree. Hope this all helps.
Cheers
Malcolm