A discussion I entered into a while ago, whilst brainstorming the plans for Melton...
I *like* letterbox view, see the Apas, it draws you in but also creates a very limited viewpoint, you almost force the viewer to look at it from said perspective. The higher the pelmet the more the view is destroyed by operators midriffs, and other body parts. Exhibition managers I have spoke too are not keen on eye level pelmets as this does not promote discussion between operators and punters.
My personal opinion is if its a low heigh layout, keep the pelmet low and have a clear view over the top of the layout to speak to people (and watch the little fingers). If its a high level layout and the pelmet will obscure your view, think about moving the main operator to the front of the layout, this is a nice touch that gives the punter a 'warmer' feeling...