How do pass/fail classes look to PhD admissions committees?
In short, this looks bad.  

When I see a "Pass" or "S" grade on a transcript that I'm evaluating, my default assumption is that the student did work equivalent to the lowest passing letter grade (e.g. a "D" in the US system.)  

I tell the undergraduate students that I advise that if they want to go on to graduate school they should avoid taking courses on the S/U grading scale.