Yorion, Sky Nomad The Companion cost for Yorion is deceptively harsh - adding extra cards to your starting deck actually hurts quite a lot, reducing your deck's consistency by a long way
Players may like it more for its novelty than sheer value
All three of those are powerful effects in a battle at any kitchen table, and you can immediately use all three
Cosima, God of the Voyage Wife of Alrund and mother of Toralf, Birgi, and Kolvori, Cosima, God of the Voyage is a strange card
Tapping them for colorless mana can have upsides with cards like Thought-Knot Seer
(MTG packs usually contain one rare per pack.) Of course, conversely, you can only get fixed rares, and you cannot get important items called Wildcards (= exchange tickets for anything) unless you open packs, so you can't do this forever, but for a while, I think it is good to play while opening packs