Karazhan is one of the best adventures Blizz has released so far, with a lot of good cards for all 9 classes. Is it worth buying... Well, maybe. If you, as a new player, are enjoying the game and feel like you are going to play a lot of time from now on, I would say it is sure a good buy even knowing it will rotate in 2018.
On the other hand, if you don't plan taking the game seriously and you are playing it just for fun in your free time, I would say buying packs is a better option, assuming you still lack a lot of the classic set cards.
All in all adventures have the best cards/money ratio in the entire game, so keep that in mind when making your decision.
PS: I hope you have understood all what I said and I apologize for all my grammar mistakes, because I'm not a native speaker and this is hard for me :P