Maybe it's the lighthearted mood and the colourful pictures and atmosphere it puts in my head. (Of course there are some darker elements, too.)
And I absolutely LOVE the Wizard of Oz part. Cracks me up every time along with many other parts.
Also the roadmovie feel and passing by different locations. (Which I find very fitting, here. )
I feel like I might start reading it again.
Out of the witches books I'd put Carpe Jugulum second, but I have to say, despite loving the witches, I wouldn't put the other books high in my list, now that I look at them. Especially Masquerade and Wyrd Sisters. (The latter perhaps because I never read MacBeth and Masquerade because it has very little rereading value for me.)