Never used them, so can't answer your questions from a personal perspective.
A video posted by Fallout Hobbies states that they will last for several uses.
As with any kind of stencil I imagine you will be able to tape them into position to use them. Obviously on intricately shaped surfaces it will be difficult to ensure a good contact between the model and the stencil, so there will be a greater chance of the paint running under the stencil.
Fallout Hobbies has got a Facebook page, so it might be a good idea to ask this question on there.