Nitinol: Useful stuff?
I've been told two things:


It gets very hot, which imposes some limitations (the person who told me this was using it in clothing).
It contracts and expands disappointingly slowly.


However, I recently came across a device developed at the (late) Haptics Laboratory at McGill University called a "high strain shape memory allow actuator":


  They consist of thin NiTi fibers woven in a counter-rotating helical pattern around supporting disks. This structure accomplishes a highly efficient transformation between force and displacement overcoming the main mechanical drawback of shape memory alloys, that of limited strain. They are as fast as the fibers are thin, while preserving the humph due to the many fibers in parallel. Because fibers can be made very thin indeed, they can twitch and relax in milliseconds. Various variable structure controllers were successful at controlling position, force and acceleration for vibration control. Thus we have high strain, high speed, high precision Shape Memory Actuators amazingly simple to build and to control.