Almost tempted to give it an extra star, but even though it's well written (though tedious in places) and indeed rather progressive for the time, it is a product of its time - and I can't get past the racist language and depictions of non white characters.
Indeed, the central message seems to be not that slavery is wrong, but that white people need to be more careful about who they enslave as some "negroes" can be *almost* as good as Europeans. I found it a difficult read - despite its short length it took me a lot longer than I expected to get through.
Unless you are interested in early English novels, this might be one to skip.