The most interesting points of flat assembler are elegant syntax, "Same source - same output" philosophy, and a very powerful macro system that consists of preprocessing and assembling stage; the fasm preprocessor has been shown to be turing-complete [0].
In 2008 I've played with it, creating macros that patch a binary file (and used it to extend a closed-source Windows program [1]), made PE format from scratch [2], a simple PE encryptor [3].
[0]: http://board.flatassembler.net/topic.php?t=6624
[1]: http://board.flatassembler.net/topic.php?t=8876