Publisher promotional freebies generally reflect the quality profile of that genre or publisher--that is, some will be bad, but it's the quality of bad that you might otherwise pay for before discovering; and some if it just won't be your cup of tea. But since they're promotional, they're of a quality such that the publisher might reasonably hope to hook some new fans in. That is, I'm not into human-alien slugfests, so I might avoid certain offerings by Baen. On the other hand, Baen has Bujold freebies, which are some of my favorite books ever. And Lackey. And Moon...
Self-pubbed freebies -- I avoid these unless the author's previously published somewhere before or I've heard something good about it. Some of them can be good but I've chosen not to search for a needle in that particular haystack myself.