Before I criticize, I should be clear: I love this show. I wouldn't be watching season 7 (or writing long critiques!) if I didn't. It's because I love this show that I hold it to a high standard, which it occasionally falls inexplicably short of.
Case in point from way back: Dean spends four decades in Hell, experiencing traumas so overwhelming that he becomes a willing torturer of souls after "only" a decade. For thirty years—roughly equal to his entire mortal life—he inflicts daily, unspeakable agonies on people, even impressing Alistair with his "skills." But when he returns to Earth, aside from a few nightmares, he seamlessly bounces right back into a life he must only recall distantly, through a forty-year blasted hellscape containing memories of nothing but (literally) hellish torture. In Buffy, Angel had to be chained to a wall and nursed back to sanity after only a few months in Hell—and he's the definition of self control. (Quite honestly, realizing this made me take a long break from the show, until the brilliantly conceived and executed "war in heaven" story arc drew me back.)
First, the good: This was great as a whimsical stand-alone episode, isolated from the character and plot arcs currently "in play." And speaking of Buffy, I loved the return of Charisma Carpenter and James Marsders to the screen. It's been way too long: they have phenomenal chemistry. I only wish they'd been in the episode earlier, with the "War of the Roses" theme taking center stage from the outset, filling the episode with deliciously scene-chewing moments of escalating villainy from them both. The writing was top-notch, too, with a lot of great lines and subtle, easily-missed humor (Dean's Google searches are a riot!)
Now, the ugly: As I said in my intro, I hate it when a stand-alone episode unnecessarily (key word here) and so completely isolates itself from the "big picture," that it totally undermines, even contradicts, existing character arcs.
Two (three?) episodes ago, Dean was so appalled at the mere possibility that Amy might kill again—Amy, the kitsune who was doing all she could to keep from killing, the kitsune who only killed out of rare and even sympathetic desperation, the kitsune who saved Sam's life by killing her own mother—that he betrays Sam to mercilessly kill her behind his brother's back. I actually agreed with Dean's decision, though not his lying to Sam: In all likelihood, she would kill again, sympathetically or not, and it was Dean's responsibility to protect those future victims who would be unable to protect themselves.
But in this episode, while still wracked by guilt over this "necessary evil", Dean never even questions the wisdom of letting two volatile, immortal, psychopathic witches carry on their merry way, despite having seen first-hand their flippant disregard for human life, despite knowing full well (one would hope) that another trail of bodies will surely follow in the wake of their next lovers' spat.
"You will kill again" were the last words Dean said to Amy, his sole justification for betraying Sam to murder her in front of her child. That observation was enough for him to betray his brother two episodes ago, but not enough to even raise the issue with Sam here (maybe so they could try again with room-temperature chicken feet). And Amy was at least trying; she was no psychopath, merely driven by desperation. These witches were clearly psychopaths with a total disregard for human life, and Dean... never even questions letting them go.
If this happened back in Season 4 or something, fine: character growth; "it's old news, fanboy, get over it;" and similar arguments might work. But Amy happened only a few episodes ago, and remains a major, "live" character thread. Indeed, it appeared in the flashback to this episode, and was clearly on the writers' minds through many of the guilt-laden scenes between Dean and Sam.
I'd also be more forgiving—especially given the whimsical tone of the episode—if "forgetting" this fact was necessary to carry the episode. But it wasn't necessary: Sam and Dean could have found a way to imprison the witches, or strip them of their powers (allowing for them to regain them down the road, if the showrunners wanted to leave open the possibility of their return as villains/reluctant allies—and wouldn't that be great!).
It wasn't necessary, it was lazy writing. This show deserves better than that.
(My original review was—entirely fairly—removed because I inappropriately addressed downvoters (not insultingly, but I appreciate the general policy all the same). My apologies, and I shan't make the same mistake again... unless I'm doing it right now. I am pretty sure this statement is okay, but if I'm wrong, I pray you'll please delete this parenthetical, if possible, rather than my entire review. Thanks!)