Used to be random enemy, now it’s minions only.
Feels bad to use, feels bad to have it be used against you. With the amount of +1 (and even +2) Spell Damage minions mage has access to, just one of those cards is enough to suppress the enemy board for a good 2-3 turns, long enough to allow the mage to develop a good enough board that the opponent can hardly fight against (speaking mostly of arena, of course).
One single card will deal an average 9 damage, split into small instances of damage, which means not only that spells like Fireball, which although strong, consume a chunk of mana at once and will often be a bit overkill, with Seabreeze Chalice you not only mostly deal exact/almost exact lethal damage on minions, but you also have an absurdly easier time with mana management.
Also, since it technically is 3 cards, spell damage works x3 on it, which is another bonus compared to your standard “Pay X, deal Y damage” card.