Do this ONLY and ONLY if you want to get the idea of how certain things are done, or something like this. DON'T COPY IT OVER!
The maps that are been decompiled get HUGE MESS in them. Looks like they were created using only the clipping tool, very badly used. Loads of invalid brushes, some brushes don't even load due to "errors in the file". Horrible!
By the way, does anyone know if Source decompilers create the same mess?