I've now tried Pepsi Holiday Spice.
It's...interesting. First, the colour is a bit odd. It's definitely more reddish-orange than normal Pepsi; held up to the light, it's almost the colour of strong iced tea. Second, it's incredibly sweet, but I'm not sure if it's any more so than the regular sugared variety, since I'm used to Diet Pepsi. Third...well, it tastes of nutmeg. Very strongly of nutmeg. There's maybe a hint more cinnamon and vanilla than usual, and if I'm not mistaken the aftertaste of essential oils seems to be more orangey than the usual lemon-limey mix. But the dominant flavour is nutmeg, and you could probably achieve the same effect with about a quarter-teaspoon in a highball glass. I imagine it'd mix well with a good dark rum like Gosling's or Lamb's.