I agree with the other reviewers as I too prefer the lighthearted episodes and Castle/Beckett banter. This episode didn't have much of that, but a sniper is a very serious matter, as is PTSD, which people do struggle with. I disagree that someone with Kate's personality wouldn't get it. That's not true. Mental health issues are far more prevalent in our culture than most realize and they can happen to anyone. It's sad that in our society, there is still a stigma attached to it any a lot of people suffer in silence.
I've never been shot or had anything worse than a minor car accident happen in my life, but I would think that any major life event can cause depression, anxiety disorder, PTSD or any number of other diseases that are just as real as any physical ailment.
I think this episode was well done. The first one of Season 4 (Rise) was all about the post-shooting Beckett and her relationship with Castle as well as an adjustment to the new Captain (I miss Capt. Montgomery more with each episode). In this show, the case was central to the backstory and in fact the cause of it.
Castle giving her the space she needs was undoubtedly very, very hard but it's what she needed. It was also nice to see Alexis and the role she played in identifying a key piece of evidence central to solving the case. I love the Rick/Alexis relationship as much as any in the show.
For me, there's no such thing as a bad Castle episode. This one wasn't the best of the season, but it was an enjoyable one.