I purchased one of these about a year ago and have it set up on my JVC DLA HD350 Projector.
Have found it is well worh the £229 that i paid, makes the picture much clearer to my eyes, it feels like an extra focus adjuster.
Go to http://www.ideal-av.co.uk/ItemPages/darbee.htm
Will give you really good examples of the benefits, is very accurate to what i have experienced in practice.