I actually used a cat kaiju against my Greyhawk players soon after that issue came out. They were battling some vampire monks in an abandoned monastery, whose responsibility had been keeping the kaiju asleep. They were only 15th level, but every now and then I put in a fight the PCs cannot win and should not attempt. Anyway, they're deep in the monastery in a room who's floor is the head of the kaiju. While they are trying to figure out what is up with this strange squishy floor with large cables coming out of it that smell like an animal, the paladin pulls out his greatsword and says "I stab the floor." That player is now forever known as Shane "I stab the floor" Patrick.
Anyway, they escape the crumbling monastery just in time, and are face to face with with a gargantuan kitten. The barbarian gets initiative, boots of speed, rage, charge. A bunch of inneffective missile fire follows, until the sorceror tries a fireball. Note that kaiju have spell reflection, so the party gets hit with the fireball. Then the kaiju takes a playful swipe, taking the barbarian down to half his hit points. The PCs proceed to flee.
The best part was while the combat was going on, my cat Hermit jumped up into one players lap. Now, Hermit is still working on that feline grace and agility thing, and almost didn't make it. She of course compensated by digging her claws in to Mike's leg, eliciting a blood curdling scream from said player. One of the best moments in the whole campaign.
So yes, kaiju can be great fun. And yes, make sure your players have enough levels before throwing one at them.