This episode definitely had a good degree of darkness for an episode where the Charmed Ones get turned into things; which are usually lighter.
Banshees: I liked the concept of the Banshee (though as usual I'm sure the writers took previous mythology and changed or added on to it). The pre-credits sequence was one that was genuinely scary and really threw you into the theme song (the first time I saw this episode anyway). The banshees have been the darkest demons on Charmed since the Grimlocks in season 1 and I liked that.
Prue: It's probably unfortunate she spent most of her penultimate episode as a dog, but she looked absolutely beautiful during her human scenes. I really miss Prue's sophisticated manner and wished she had stuck around to yell more at Phoebe for defacing the Book of Shadows with her self pity scribbles about Cole. Prue seems a lot more mild mannered in this episode, though probably because she knew Phoebe was in pain so she let her off more easily.
Phoebe: When I first watched this episode I thought the concept of turning Phoebe into a Banshee to work out her intense pain was a well executed idea. The banshee was played well by Phoebe though I didn't like Phoebe in general in this episode. I know I have to separate Alyssa from her character, but she gives Prue/Shannen all of these dirty looks and attitude throughout the episode that is just hard to dismiss. Prue lets her off the hook too easily and goes along with her dumb "Belthazor is behind this" theory about the next victim; so Phoebe has no right to take it out on her.
Piper: Piper features some of the funnier and snippier lines in this episode ("huh, shut her up!") that really carried this somewhat dark episode. Still dealing with her witchy inadequacies ("I just tag along and freeze things"), Leo encourages her into believing she's just as powerful as Prue and Phoebe. I don't know why she ever thought she was less powerful than a premonition crap witch who can fly with strings, but ok, maybe it was a reference to her obscurity behind the Shannen/Alyssa diva scenes (sorry, I'm biased).
Cole: Cole's another funny character and you have to love his face when he sees Prue-dog and Piper's botched vanquish. The fight scene between him and banshee Phoebe make the episode as well.
Conclusion: One of the best "turning into things" episode mainly because of the dark back story behind banshees and Piper being on her own to figure stuff out. It also sets up the stage for All Hell Breaks Loose since by the end of the episode a concerned Piper at P3 looks on with a thoughtful and concerned gaze as Phoebe joyfully becomes determined to save Cole. Unfortunately that quest may cost both her sisters lives in the pivotal episode that follows.