The FreeBSD installer should be just like slack (its been a while since i last used). It will wipe our your slack partition and replace it with BSD. FreeBSD can be used as a desktop OS, however some of the most popular linux apps arn't supported for BSD. BSD is UNIX, not LINUX. Its just Free UNIX.
Here was my experience with FreeBSD. It looks similar, tastes similar, but feels differen't. Its kind of like wearing someone elses underwear. It serves the same purpose, but its just not the feel for me.