I am currently an Instructional Math Coach in Colorado, but I have taught out of Core Plus. In both my teaching and my coaching experiences I have found Core Plus to be very useful in creating the kind of problem based learning environment described by the CCSS Standards of Mathematical Practice.
The problems in Core Plus are rich and rigorous, but the program is not a silver bullet to getting your students to adequate levels of proficiency. It must be coupled with the instructional practices that maintain the richness and rigor of the program. As mentioned in previous posts, Core Plus is the best program available to implement Best Instructional Practices and so I highly recommend it for the development of proficient students. However, proficiency comes from the instruction, not from the resource.