You have to be fond of the series to enjoy this. That's pretty much it. It's a retelling of the first season through Reigen's very altered point of view, so if you like Reigen and can get past the fact that he manipulates kids (as I can) then you'll enjoy the special. Because this is very much about Reigen and how he attempts to manipulate people. It's also a recap, so if you don't want to go through a reconstitution of season one, don't bother.
I enjoyed this. I love Reigen; he's my favourite character in the series. I also needed a recap on this (I don't have the best memory) so I was fine with it. Again, it's really about personal taste. If you don't like Mob Psycho very much, you're not gonna like this. 'Cause the art, sound, character, story, etc. are on the same level as the series, obviously.
I'm thrilled about the second season of Mob Psycho 100, and I can't wait to see what's in store.