Hey Lestat, i know of one place that uses one here in maine, they are a small lunch kitchen.
Since seeing his set up i have been looking into this myself as a cost saver for start up.
His is used over an electric griddle, has full ansi and the owner opted for it over the fully vented because of cost. It has a special filtration system that fulfills local HD requirements and the necessary ANSI system for fire code.
So for a low cost option if you can get it cleared you will save some dough, he got his for under a grand, but most i see are for more the 2500 used. His is a shade over 2ft in width and he has a 24" griddle running below it.
Still cheaper than many vented hood systems. he also has a self fryer with a similar system that operates in the same room.
in maine they can be used so long as the have ansi and (i believe) so long as it is for a fryer or electric cook top.
<message edited by hella good tacos on Wed, 01/23/13 8:16 AM>