Hi! I just thought I'd chime in with my 2 cents.......
While I haven't actually done CC, a good friend of mine has and has tutored. She's given me the complete run-down and I've visited a CC group for a day to get a good feel for it.
Her advice to me was that if you want the long term benefit of CC, it is so beneficial to start it when they are young and stick with it. (I suppose you could start later, say, in middle school, but you wouldn't have that same foundation) As you may know, by memorizing all the facts, you are creating "pegs of knowledge" that you will later add understanding to. The classes look fun, and the children seem to really enjoy it. I wouldn't think that kids would enjoy rote memorization, but somehow with CC they actually do seem to find it fun! Then there is the social aspect, and that is really my main temptation to join, as our group has some amazing godly families that I would love to be around more and have my kids around as well.
However, with all of that said, I do have my concerns...which echo what some of the others have said. I prefer the gentle art of learning that a CM education provides and I prefer narration to help knowledge stick over rote memorization. Also, this will be our first year of homeschooling, and I want to make sure that I don't overload our weeks. It's most important to me this year to have very light commitments, not much at all outside of church things. Also, my daughter has other avenues right now for social interaction, and besides, I don't think she really needs that much outside of our family at the age of 7.
After considering these things and praying about, (and talking about it to DH a ton!) I don't think we'll do it this year. After this year, I may take another look at CC and see if there is a need for more social interaction. If we did do it though, I think I would pretty much do what burtmommie said she does...I would try to stick with our CM plan and do CC as much or little as I felt appropriate.
HTH, and sorry if that is more info than you wanted, that was just my process for considering whether or not to join. Remember - pray about all things...Philippians 4:6-7