Our kid's experience seems to confirm what Easton states re the -10 Synergy:
no change, except cosmetics (color), from 2006 to 2007 to 2008.
She is very happy with the 2008 (lighter blue with grey lettering) she got this tournament season, especially because she says it feels the same as her old black 2005 Synergy, and the 2006 or 2007 team Synergy bat that she used the past high school season.
Not sure whether they still make a -9, but that was a very different bat (and not high on our kid's list).