As long as I am entertained by the book, I don't care to which degree the writer added their own persona into the main or side character. Story-telling naturally starts with 'that cool, incredible thing that happened to me' and tbh, I think most people would tell a more organic story when they let themselves to be close to their characters.
There is one type of self-insert that I don't like, the Pity-Sue, when the writer re-imagines themselves as some pitiful abused maven (for no fault of her own) to be saved by the Manly Man from their misery. I prefer the all-conquering version.
Overall, self-inserts land themselves well to the stories with bright, bold strokes and huge drama of passions and end of the world and what not. And, those stories are fun to read.
The success depends on how much the alter-egos links to others and how the writer balances them with the rest of the cast. Usually somewhere mid-way between the two extremes is neat... and putting in more than one self-insert is also a good idea, like antagonist and protagonist. It helps with oversimplifying each character, and avoiding the situations when either everyone loves or hates a character in the cast...
All and all, self-inserts are perfectly fine as long as they are fun. And, psst, nobody needs to know!