a spoiler free reason? Monster Hunter isn't one of them story driven games, it's mechanics driven.
One of those games that you despise as soon as you start playing but you can't put down, you can't even think about putting it down. I've been playing the game ever since it hit the PSP and let me tell you, 1500+ hours clocked in and I still love the games.
As a plus, MH4U is "newbie friendly", it isn't as infuriating as the Freedom games and to finally answer your question, yes, Monster Hunter is worth it especially if you've got a few people willing to play.