With a unique effect that puts every banished card right back into the deck and then adds to the monster's ATK, it was a great card that was either limited or semi-limited for much of the game's life until fairly recently
The big exception was a $5,000 Neutronium pack opened near the end of the stream, which resulted in a $7,000 Rayquaza card
Mass of Mysteries, from Lorwyn Eclipsed Commander , is arguably the weakest of the give myriad to other cards, but it's still very good when giving myriad to elementals with strong enters effects, like Mulldrifter
They make it easy to build decks without spending a fortune on packs or single cards
Between prices for cards going through the roof and older cards no longer having any use in the game because of power creep, Yu-Gi-Oh has quickly become a mess in desperate need of fixing
I have a huge collection - can I bring it all in at once