It was hit and miss. My least favorite was the making of special followed by Welcome to Phantomville. Both were kind of throw away and I just watched them to see if they were worth my time. Sadly, they weren't.
Both of the Wonderland episodes were funny and satisfying to watch, though the second episode was leaps better than the first. Grell as the Cheshire Cat was fucking brilliant so points there. I'd give the Wonderland episodes as a whole 6/10.
its when you get to the other two that the real treat begins. The add-on episode about the "Spider" clan was amazing. I loved it so much. Alois Trancy is such a broken character and I loved his story so having a look inside the heads of Claude and Hannah were interesting.
Lastly, the best stand out was The Story of Will the Reaper. It's basically Grell's life during his first reaping. It's a great watch. I loveeeee Grell so going into his backstory was cool and exciting. This is the gem of the specials.
Overall, the good outweighs the bad and makes the specials a worthy watch for fans.
Watched - dubbed