Noblesse:Rai's adventure is really incomplete and a bit boring after you have read Noblesse. It really didn't bring out much emotions from me. It's timeline is set before Frankenstein makes his entrance and Getuchel and The Lord are the only other characters we have known previously. It doesn't even have much of the humour that Noblesse manhwa is known for. Read it if u want but it doesn't matter if you don't, you won't miss anything.
This is what it would look like if noblesse made it as a hit anime series(not likely tho given the ratings) and produces a spin-off movie franchise like naruto. In other words, it's okay-ish, no mind-boggling events happening or anything; just Rai stepping out of his mansion and reverses expectations of his foes in the end like always. Very much like naruto movies, new set of characters, same display of skills, defeats villain, and the end.