I've liked Sableye for some time, too. I first liked it for the novelty of having no weaknesses (and I used it in the battle against Brawly in Emerald, where none of his Pokémon could touch me). Soon I fell for its hilarious design. Then I say the Sableye in Pokémon Mystery Dungeon: Explorers of Sky and became even more fond of him. Then he became the troll with Prankster that everyone knows and loves. Now... Mega Sableye. New design, new stats, new Ability... new game.
Not only do I like the design, but I've been pleasantly surprised by its new dimensions in competitive play. I actually think that he has the potential to become a monster with his new Ability, given the right stat distribution. Even if he sucked, I'd find SOME way to use him. He's one of my favorite Dark types and possibly my favorite Ghost type; also, regardless of how his Mega goes down, his regular form is still viable.