In my experience very very bad value. I signed up for physics with the NEC. The materials were interesting and well written but DID NOT ACCORD WITH THE SYLLABUS THAT WAS SUPPOSEDLY BEING COVERED. There were a lot of bits in the NEC notes that weren't requried, and a lot of stuff required that WAS required that was nowhere to be found in the NEC stuff. Furthermore even the stuff that did cover the required syllabus provides NO preparation whatsoever for the eventual examination. You do about six problem sets during the course of your studies that are marked by your tutor, but these problem sets do not in the slightest reflect the style or difficult of questions on the actual exam.
Overall it just seemed as though somebody at the NEC put together an interesting physics course without ever bothering to check whether anybody could actually use it to pass an A-level. What I eventually ended up doing was to buy the Nelson Advanced Science Edexcel study guides and go through loads of past papers.