Headdoc, I loved Core at first but switched to Flex, which I hadn't been on since last year (no WW at all) for various reasons ... mostly that Core is too restrictive for me and the plan seems inconsistent. But I'm still interested in Core as an option for switching back and forth for the very reason you mention: plateau busting. I think WW's "Turnaround" concept is a good one ... switching the plans occasionally seems like a good way to keep things moving.
Just a comment.