I have that kit on one fj40 and it rides pretty good and hasn't sagged. For the money it's not bad.
While most any thing can be compared to another thing, in a comparison of liftkits the Black Diamond kit would be found to be very different than a liftkit from Old Man Emu in several relevant aspects.
Have you seen the FAQ section in a thread at the top of this forum? A user called "poser" has done an outstanding job of bringing together previous postings in this forum on several subjects including liftkits.
He has made the vast storehouse of accumulated experience of many users available to all who come here - available with no waiting.