From all the external render engines available for Blender 2.5, LuxRender is my choice for projects with internal visualization of architecture. The quality we can achieve is amazing, with only a few mouse clicks and the use of some materials presets. Of course, I do recommend you to study and test the materials to be able to make all proper adjustments to the materials for your own project. If you want to add a few tips about LuxRender materials to your roll of knowledge, a new section was added to the LuxRender Wiki showing how to work with the Car Paint material.
Is this useful for architectural visualization? Sure! Just think about any metal material that should look like the car paint material, which is not hard to find on commercial interior, especially stores and restaurants.
The entire article is based on a set of experiments created by PeterLux at the LuxRender user forums. The article is organized with lots of images rendered along with the settings for the material, and explanations from the author about the effect that each parameter has on the final result.
If you use LuxRender, go read it!
