I enjoy them for the most part. Although when It's clear that it's just following the same tedious steps such as: pulled from earth via portal or magic, Immediately setting up a friendship with the mane 6, Gets told they can't go home, Protagonist acts frustrated or sad, the pity party begins etc. It just has to at least seem like it Isn't just the writer's fantasy.
Self inserts don't bother as it does with others as I get why some want to write one, they just has to provide an interesting and reasonable story. For example a story where the human feels like they have little to no choice but to become a villain to achieve their goal whatever that may be, and aren't scared to stab the ponies in the back rather then following the usual pattern.