I used a mach3 style razor for many years to shave my ex wife. As with any shaving, use a good lather, a sharp razor and take your time. Oh, and plenty of moisturiser afterwards.
One of my best friends shaves his girlfriend with his straight, it's good for concentrating the mind she says!!
My current has expressed an interest in me shaving her with the straight instead of a regular razor, but that's a fair way off as I'm only just starting to shave my face with one. Once I'm comfortable with it though, I think it'll be a worthwhile experiment.
J.