I really enjoyed this product. I really think it fits well with a sort of espionage or political intrigue game as the ratfolk love their secrets. I can see an entire thieves guild being run by the the Rat King or a wererat. Its low level focus makes it a great way to introduce the characters into secrets, lies and hidden motives. I like that the ratking can buff his minions and unleash psychic attacks. I hope to see more like this.
The only concern i really see is a CR 1 having access to multiattack via a legendary action. I feel this should be a more support feature as it puts its damage pretty high up there for a CR 1. According to the DMG a CR 1 monster shouldn't really be doing more than 9-14 damage per round. Both the mind blast and the short sword do 4d6 each. so 8d6 or 28 damage per round is right up there with a CR 5. I do recommend this product, as a DM I would just be weary of the damage calculation of the monsters.
[4 of 5 Stars!]