Creating High-Quality Maps in Source
Important:Help Wanted! Editors with experience in this area need to contribute to this article to keep it accurate.
Many recent high-quality 
Source maps (e.g. 
Black Mesa's Xen levels, Cascade (de_prime), and de_ancient) have very complex and intricate geometry across the entire level. While Hammer can create similar geometry seen in the Antlion Caves in Half-Life 2: Episode Two, Displacements can only accomplish so much before reaching their limitations. The answer to how these maps are created is simple: Models. Created using superior third party modeling programs (e.g. 
Blender), small parts of levels are modeled and then combined in Hammer to create a very high-quality and detailed map.
Todo: Put documentation here. While this is still being worked on, view this page to see how to port blender models to Source
Theory
Note:Remeber to review Basic Blender-to-Source guides before continuing. You must have a good knowledge of Blender.Basically, what you are going to be doing is creating the complex sections and models to decorate the map and make it appear more realistic. While most maps do include models, these are usually reserved for select high-quality props, while the majority of the map is made using Brushes and Displacements. However, with games like Black Mesa, sections of the map that need to be higher quality than what Hammer can make are modeled in third-party programs and exported to Source in its model format. This is only half the battle, however, as models will still need to be manually positioned and lined up. Remember when using displacements in Blender that most plugins use a 1:32 Blender unit to Hammer unit scale (see Exporting displacements from Blender). For newer games like 
Counter-Strike: Global Offensive, this is less of a problem as those game have a model static prop scaling feature. Also, keep in mind that you should not abandon normal Source methods of level creation entirely; some flat or slightly curved surfaces can still be made using brushes and displacements, and having to recompile an entire model and subsequently an entire map can take a very long time, especially if you need to just make a small adjustment. Instead you should get the general map layout perfected first before even opening Blender.
Using Models when Appropriate
Think of de_ancient in Counter-Strike: Global Offensive, with its very high-quality exteriors with lots of detail and finish while many inside areas and floors look somewhat more basic in comparison. This is because the map uses a great balance of Brushes/Displacements and Models. Surfaces that the player may not be looking at a lot or that just simply don't need as heavy details can settle for Displacements or brushes. It is heavily recommended, if possible, to use PBR shaders if available as they can give added depth and realism to flat surfaces that Normal Mapping cannot do, which can help to improve the look of these more basic surfaces. However, that is not to say that indoor areas can't be models, as, for example, caves and the like have very detailed interiors that usually cannot be created with normal Source methods.
Downsides of using models
Todo:
Note:This article is not complete! Feel free to add information if you would like