I posted a link to the Wikipedia article about it, but yeah it is a culturally significant drink over Christmas, there is no organized boycott of Coca Cola, but I suspect that people enjoy the fact that Coca Cola tanks their sales in Sweden during christmas.

There are several versions of Julmust, there is one company making the syrup, selling it to drinks manufacturers, who all tweaks it to their preference.

The most common/standard version of Julmust is "Apotekarnes Julmust", though "Nygårda Julmust" has expanded a lot, there is one version that I didn't enjoy when I was younger, but is my favourite these days, "Zeunerts Julmust".

The taste of Apotekarners Julmust is quite hard to describe, the texture isn't as harsh as Coke, it is less sweet, and more fruity, it is a more adult taste than Coke, dryer...

The Zeunerts version is quite odd for the uninitiated, it has a darker taste when compared to Apotekarnes, with a weird aftertaste of coffee. When I first had it, I didn't like it at all, but these days I stock up on it.