There are normally 2 problems. Firstly, the SI is favored more than the other characters of the story which makes it seem like blatant wish fulfillment and secondly, that characters become so different that they are unrecognizable.

 

This answer, it all really comes down to the style and composition/quality of the writing.

 

Not necessary. After all, I have read some good ones. Usually I try to restrain from making judgments until I have at least read the stories. However I will expect major divergence from canon. At the very least different conversations than those used in the original stories.

 

Personally, I have yet to read a good SI fic. They just have that habit of being so likely to break the suspension of disbelief, and just pull you right out of the zone. They too often come across as said earlier as "blatant wish fulfilment", and often read like a fantasy the author has had where they thought, "My life would be so much better if I was a character in Harry Potter".

 

Self-inserts aren't necessarily bad, though I would never be able to complete that challenge seeing as
A) I'm a terribly uninteresting person and it would turn out more like a
Wish Fulfillment which I believe are inherently bad; and
B) I'm an awful writer.

It really does depend on the author and how true they are to themselves and the story. It would be cool to see one where they inserted themselves except lacking the knowledge of the final book or if they only had seen the movies or had only read fanfiction.

I actually have read one really good self-insert, Magical Me by Publicola, which is good because it's humorous in the sense that the author wakes up as Gilderoy Lockhart and interesting because he goes in depth about magic and certain aspects of magic. Unfortunately, it's unfinished.

 

Don't know if its just me, but I have yet to read a good self-insert fic

 

I just don't enjoy self-insert fics. Despite the fact that the vast majority of them are absolutely horrid, even the well-written ones wouldn't hold my interest.

It's like a Harry/Millicent pairing, a fic might be well planned, thought out, greatly written - but it just wouldn't make me stay for more.