For rules, its worthless. For fluff, its invaluable.
I dare say it had the best fluff written until the 3rd ed necron codex was printed,
IMO.
The witchhunters codex only barely hints at was was in the 2nd ed sob codex. A brief outline of the Goge vandire's incident and the age of apostacy.
But Wiki, as usual is your friend.
en.wikipedia.org/wiki/Lord_Vandire I havent crossed checked to make sure everything is there, but it appears that everything in the 2nd ed sisters codex and more is in that link.