Slaking: just cool or even useful?

Novelty.

As useful as Ho-oh and Suicune.

Slaking as an attacker = yawn

It might end up being an interesting defender. As an attacker... CzarJG24 said it best.

A gym with Blissey/Gardevoir/Snorlax/Metagross/Slaking/Dragonite would be an absolute nightmare to deal with.

Junk. I would gladly take my 300 rare candies and 200k dust back.

There is something called blissey in the game which is broken af
Slaking is not even close...

As a defender, unless it has its 2-bar fairy attack, it likely won't even do any damage before being knocked out.