Let's see here. Quite a few of these have been used against me, and I've used several.
Jovocs and palrathee have both showed up in a game I play in, as written. I've also based some undead heavily on the jovoc for my game (weeping children, a type of undead that reflects damage it takes as negative energy damage, healing other undead nearby).
Fihyrs were a major enemy in the beginning of my current Rokugan game, with an enemy of the PCs seeding the dreams of a beseiged town with items that made the creation of fihyrs more frequent. These things showed up many times, for about two months of gaming.
Meenlocks were the subject of the Eberron demo game I ran a few months ago. The plot was that a cultist of the Dragon Below was trying to poison Sharn's water supply with a potion that would make people who drank it turn into meenlocks. They only showed up once, but it was a fun encounter.
I had a morkoth encounter all planned out for a recent sea voyage, but the PCs bypassed it through some clever diplomacy, detective work, and divination magic. Good for them.
I've had a nimblewright used against me as an assassin sent after a prince we were supposed to protect. It's amazing how hard it is to fight something if you don't know it's a construct.
I used a group of sirine as a random encounter recently, while my PCs were traveling beside a mist-shrouded mountain lake.
I tossed some advanced, Corrupted (from the BoVD) twig blights in with a treant encounter not too long ago. They got munched very quickly by the samurai with Great Cleave and a flaming katana.
The chimeric and tauric templates were used when the PCs went up against a fleshcrafting Chuda bloodspeaker not too long ago.
All in all, I'd say I've gotten my money's worth out of the book. There's some good stuff in there. I'm more impressed with it than I am with MM3.