I've only ever used one as an antagonist for the PCs of my game.
A significant henchman of the campaign's main villains, the nefarious Dr. Braumstein von Gleamingwand was an enemy the players still talk about.
I modelled him after the typical mad scientist/kooky surgeon idea. Most of his spells were fear inducing, enchantment and other "mind warping" effects. There was a Dragon Magazine that had an article full of spells from one of the mad scientist characters from Ravenloft that was very fitting.
I picked Beholder grafts for him because I liked the idea of big bulging eyes and gaze attack like effects (I went with sleep.. and it did a number on the party).
One of the players couldn't make it to game for a small period of sessions, so I had his character just go missing. When he finally came back, the character had to escape from the mad doctor's lab, after having been abducted and experimented on so the villains could get a one-up on the party.
But, the hands down, best thing about using this type of character was all the weird and messed up "experiments" I could send at the party. Misshapen monsters, people and other animals with weird grafts.. all of it designed to freak the party right out.. and it worked.
So the good news is that I have used the Fleshwarper and it was an awesome experience. The bad news is I did so in a DM capacity - I don't know how much would transfer over to the PC side of the screen. Though I will say, grafts are very expensive (and for a good reason) so having a way to reduce the cost is always a plus.
J from Three Haligonians