gcc 3.x had #pragma once bugs related to symbolic links. MSVC had bugs related to filenames that only differed by case. Modern gcc and MSVC can recognize and optimize for the #include guard pattern. Many benchmarks show no build time improvement of traditional #include guards vs #pragma once.

I some one benchmark where the developer #included all his headers and source files into a single .cpp file. Compiling this single .cpp file was about 50x faster than a traditional build! <:) Of course, you will likely have name and dependency conflicts.