So, I've never been able to finish it, but I got pretty far. This is honestly not a bad fic (and it is very popular), but I just couldn't get through it. I found the writing to be really matter-of-fact and sort of emotionless, and I didn't find the plotting to be especially engaging. Harry is also kind of OP in this. I think it has its upsides, but I've read many mentor!Snape fics that I felt did a much better job capturing the complexities of their relationship.
One of my favorite stories and one of the seminal pieces of fanfiction that, at least in my opinion, turned the community fully onto Mentor!Snape as well as Sevitus. This story is well written and one of the classics of the Harry Potter fandom. Currently rated 13th in Favorites, this fanfiction rightfully deserves all the praise it gets.