While its not quite as popular as Vivi Ornitier, Kefka has a rich competitive history, consistently topping events
For 7 mana I want game winning spells, and Mind's Dilation can be that but it's risky and not guaranteed
Of the 10 decks that dropped in price, the average decline was 13.2% (when including Dredge, it drops to a still respectable 10.8%), ranging from a high of 22.1% for Jund, which had multiple expensive reprints in Ultimate Masters (primarily Liliana of the Veil and Tarmogoyf), to a low of 4.3% from Hardened Scales (which didn't get many reprints at all)
Her soft design stands out among other trainer cards, making her a staple in graded trainer collections
What makes the game really special is the Shandalar adventure game which gives you an excuse to keep on playing