Good volume. Kind of hit and miss, though. The first chapter, for instance, seems really phoned in due to constant art hiccups along with many of the jokes falling flat and uncharacteristic dialogue. #2 is an improvement, but still below par IMO - no big laughs and very little wit. Thankfully, missions 3-5 are excellent (5 being my favorite), with many hilarious scenes and some great character interaction. One part of chapter 5 made me put the book down since I was laughing so much! Chapter 6 is also good, but not that funny. It's a serious chapter, but more interesting than usual. I can't tell you why it works so well though, because that would be a spoiler.
As for characters, I felt that Iwata and Watanabe were far and away the highlights of this volume. Perhaps not in the way you'd expect, but I guarantee that longtime readers will love them in this installment.
Misaki and Kabapu have some great moments as well, while Elgala and Sumiyoshi are as charming as ever. I didn't much care for Excel in this volume, but I do believe that may have been intentional on Rikdo's part (and I'll say no more!).
I'd probably put this one near the bottom of my list, but it's still good. None of the volumes are actually bad, after all. There's fun to be had, but it seems like there's too much plot and not enough jokes throughout, whereas books 19-23 struck a healthy balance between both, making them some of my all time favorites. Though it's worth noting that I didn't really like #21 on my first reading, so maybe this one will improve upon re-reads. We shall see!