I have some small idea of UDK.
For UDK architecture has many advantages not, UDK spend time to understand, besides the library source is available again modest, and can make available materials, effects of weather, trees, vehicles, persons assigned availability of visual motion and almost not there. This may be why many arch is not much interested.
Design time works pretty much lost, although a realtime UDK 100% but the conversion process and review has not been easy.
Light and the calibration process still takes much time and besides that to make the light look more realistic look to much effort. But the advantage of being able to create markers for each drawing.
% for each product is high.
Free or perhaps because it needed for what has been hidden architectural interesting!!
PS: I do not know English, I guess there will be errors in the language, people ignore ^ ^.!!