Its hand-written parser makes it much faster than GAS, though of course, it doesn't support three bazillion different architectures. It has mostly Intel-syntax, though it differs slightly as for addressing modes. But I advise no one to use it for anything else but compiling Linux. This version supports only a hacked minix object file format, which is not supported by the GNU binutils or anything, and it has a few bugs in 32-bit mode, so you really should better keep it only for compiling Linux. The docs are what is included in the bcc package.
Man pages are also available somewhere on the FreeBSD site.
It is said that there was a free clone somewhere, that was abandonned after worthless pretenses that the syntax would be owned by the original author, and that I invite you to take over, in case the syntax interests you.