The physics of them is pretty interesting. The slug is 'solid' until is hits a more solid object and then the mass literally explodes inside that object creating a massive wound channel unlike a traditional single chunk of lead. The energy transfer is unreal. The higher the mass of the projectile, the better this works also which is why light, super fast frangible bullets do not perform very well.
Still, until I see a vid or read a good write up on how they actually perform on big game, the jury is still out.