I would also use Magic Lantern. I have used stable releases and nightly builds
on a 550D and never had a problem. Moreover, I think it is far more convenient
to be able to set all your timelapse parameters from the camera controls, instead
of using a circuit board that you will probably fire as you close the housing.
Regarding 1D-X, you will probably need to go for an external circuit as there is
no "hack" or firmware overlay.
- Jeffrey Honda likes this