I took a body pump class for 10 months. We did all kinds of planks all the time. Over those 10 months I went from not being able to do them on elbows and toes to being able to do them for the entire duration we were supposed to hold them. So I did improve my core strength.
However, in a month of doing squats and deadlifts with heavy barbells I have more than doubled the amount of time I can hold a plank. In fact, I don't know how long I could hold one as I've never tried to do one to failure. I do planks now as both a test of my progress and as something to do to get me out of my office chair for a couple of minutes.
I still think they are better than sit-ups, though.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 192.5lbs, press 75lbs and deadlift 210lbs