Well, this was a lot better than I expected!
The series itself has tons of plotholes, lack of development, and factors that made it less interesting as a whole. However, since this special episode isn't weighed down by the downfalls of the series, it's actually really enjoyable.
First off, the story is simple enough and easy to follow, so it makes it pretty easy to watch as long as you know what cosplay and airsoft are.
The art is gorgeous like usual, and extra work was put into the 'cosplay' costumes.
The characters weren't super developed (it's like twenty minutes, what do you expect?) but
I did appreciate the little insight into Midori's character and how he wants to still work with people despite his sadistic personality. He didn't betray Hotaru at the last second, at the very least.
I really enjoyed this, to be honest, and I kinda loved the fan service with Matsuoka; it was pretty adorable to see him so ticked off. As usual, I adored Yuki and his pervertedness, along with his new photo collection. I wish we had found out how Hotaru's gender confession turned out, but I suppose that it was better off left to the imagination.
In the end, even if you didn't enjoy the actual series all that much, I would recommend this just because it's a lot of laughs and it's pretty cute :)