I'm not sure how the admissions people will view it (most likely it will help) but I can tell you from experience it makes a massive difference to yourself during the program. I entered my program straight out of undergrad. My classmates who came in with masters degrees had a far more sophisticated perspective on the coursework, and while I was struggling to learn as much as possible in preparation for qualifiers, they were already meeting with their future advisers to start preparing for research in their chosen fields.
It wouldn't surprise me if admissions committees viewed a masters degree very favorably. It would seem like the students with masters are far more likely to finish the PhD. students straight out of undergrad are a little more prone to changing their minds.