The latest, December 2006 issue of the American Mathematical Monthly, an official publication of the Mathematical Association of America, contains an 18-page article entitled “A study of Core-Plus students attending Michigan State University” by Richard Hill and Thomas Parker, professors at MSU who teach pre-service high school math teachers.
They state that, “as the implementation progressed, from 1996 to 1999, Core-Plus students placed into, and enrolled in, increasingly lower level courses; this downward trend is statistically robust (p<.0005). The percentages of students who (eventually) passed a technical calculus course show a statistically significant (p<.005) decline averaging 27 percent a year; this trend is accompanied by an obvious and statistically significant increase in percentages of students who placed into low-level and remedial algebra courses. The grades the Core-Plus students earned in their university mathematics courses are also below average, except for a small group of top students. ACT scores suggest the existence but not the severity of these trends."
Core-Plus is used in some Madison High Schools. Much more on math here.