My family is in its third year doing CC and overall its been a very positive experience. I've done both the Challenge (junior and high school programs) and Foundations so I'll offer my perspective on both.
My dd started in the 6th grade(last year of Foundations) so we used CC as our spine - but many families don't. She is a social butterfly and enjoyed the interaction with the other students in the class. It really helped her to want to be prepared for class. That year I was pregnant with my third child, so CC really helped keep us on track although so weeks were harder than others. However, I was never pressured to keep up with the group from week to week. CC really emphasizes each family going at their own pace. The students went at different paces in each subject. That year my daughter also did Essentials, which is a separate program for Language Arts, Writing, and Mental Math. This was EXCELLENT! My daughter came in a struggling writing with very basic language arts skills and by the end of the year, she no longer cried when she had to write a paper and her Standford 10 scores in Language Arts were in the upper 90th percentile.
The Challenge program I've learned to pick and chose what we will do because unlike the Foundations program, it actually uses various curriculum that may or may not work well for your child. For example, my daughter and I hated Latin's Not So Tough in Challenge A, so we dropped that class and I bought First Form Latin to us at home. But CC Challenge is flexible, like most other co-ops you can choose which courses you want to do there and which you'll do at home or elsewhere. But the opportunity for my dialect/rhetoric child to interact, discuss novels, present papers and debate
with other students is something I can't replicate at home and I like the structure that CC provides. Now honestly, I do believe that TOG is a more rigorous program and that will be our spine next year for history and maybe literature, but my daughter will still do some CC Challenges classes - Science, Economics, Latin (Henle 1), etc, for the reasons I stated above.
This year my ds5 is doing foundations and he loves it! Although our spine is now Tapestry of Grace, I find that CC introduces him to topics that we touch on later on in TOG. CC gives us a peg of knowledge that he can later hang more information on. I don't stress connections with him because thats not the purpose of CC at his level although sometimes he makes them on his own later on in the year as we study CC topics in TOG. I'm interested in immersing him with facts of information, which CC does well. He also does Science experiments and art activities there, which is a plus for us.
I have found that THe Story of the World on Audio has tremendous benefits for CC users because the CC History Sentences are also topics in SOTW. My son has reached many of his connections that way. Example, a few weeks ago we learn a memory sentence about The French Revolution and Reign of Terror. It didn't mean much to my son then, but when he heard the references on SOTW audio book, he head perked up, he smiled and said, "Oh, I know about this and broke into a rendition of his CC History Sentence. Then he had me replay the STOW chapter and listened intently to the story. As we progress through CC he'll learn more and more. This has happened time and time again with CC - hanging pegs of knowledge helps the students make connections later on. That moment when things click is priceless.
Hope this helps - this is just one homeschool mom's perspective :)