In my opinion, it's worth buying as soon as possible. I bought it as soon as it came out i don't regret it at all. It is very useful when you go hunting for more than 2 monsters. For example :
• If you care about experience and you hunt for Feru seal in the team and set :
Grimeleech
Plaguesmith
Hellflayer
and for each monster you have set full exp with 40% more exp then it really makes a big difference.
I recommend setting all 3 preys to exp 40%, when you activate and use them all, try not to use them to the end!
Leave a few minutes and roll the monsters, then you will not have to use cards every time!
You will save a lot of cards in this way so you can spend cards (5 pieces) on blocking a given monster, roll every day for free or for cash without losing 40% exp on prey. This also applies to prey on loot or def.
I recommend this way 