Monster Boar?

[Lingchih]

I've had it cast on me multiple times in an SP game recently. The thing is incredibly easy to spot (10 pd will do it), and also incredibly easy to kill (it is just a pig after all). Generally my Harpies in the PD would kill it the first turn of the battle.

Definitely not worth the 10 gems it costs.

[Juzza]

you can check how much damage it does to your enemy by checking there income on the score graphs.

[Edratman]

I've been getting it cast on me regularly in my current game. It is barely a bother. I'll never use it myself. It might be tougher if the AI repeatedly cast it on the same province, but I seriously doubt I'll ever waste the gems on it.