As any distribution it depends on your taste and convenience. This is why there are so many - peoples tastes and circumstances of convenience vary greatly.
To be specific about Slackware I think people who enjoy it like its lack of branding, conservatism in package management and configuration tools and the focus on package stability (users are assumed to dislike new features, unless they consent explicitly to them.). People who dislike it probably do for the same reasons.
Also the community is important - tastes and convenience are acquired through friends and the environment you are in.
To be specific about Slackware I think people who enjoy it like its lack of branding, conservatism in package management and configuration tools and the focus on package stability (users are assumed to dislike new features, unless they consent explicitly to them.). People who dislike it probably do for the same reasons.
Also the community is important - tastes and convenience are acquired through friends and the environment you are in.