Warning: Rant ahead!
Why is Cole a bad guy? THINK about it. Why does Phoebe hate him SO much? Paige tells Cole that he put Phoebe through so much (last season). That's fair enough...except that wasn't Cole, was it? The Seer tricked a human Cole into taking in the Source so he could save his girlfriend from certain death. Ammiright? When they vanquished Source-Cole, Human-Cole was sent to some kind of demon limbo since he wasn't actually a demon, right? He acquired some powers, but he was human. He somehow made his way back to Phoebe who completely freaks out for no reason whatsoever and decides to become a mermaid. So Cole IS human, but has demonic powers. He's basically a man-witch. A warlock. He's not evil at all as far as I'm concerned. Phoebe even tells him she still loves him but cannot go back to him. WHY? I.Just.Don't.Get.It.
End of rant.
The rest of the episode is a serviceable piece of fluff. Piper's silly subplot turns into something of moderate interest when she's electrocuted by means of a very elaborate set-piece (nice touch with the eeels). I was also impressed by the messy vanquish - all three sisters are tossed all over the place, and end up in a more dire situation once the demon is killed.
And the ending, admittedly, was unashamedly sweet, with Patty magically signing Piper's old baby book. D'aww.