One of the greatest things cardio kickboxing did was motivate me.  Several years ago, I began my fitness journey with Tae Bo, and I've never looked back since.  I found the movements to be incredibly fun, and they even made me feel powerful.  What's more, getting in shape w/ cardio kickboxing is what got me interested in other types of workouts and eventually led me to Cathe.  
As far as the physical benefits, like many of the other post-ers, I think kickbox routines have made the most difference in my abs (slimming the waistline), shoulders, and triceps (mostly toning these two groups, though I think I've experienced some strengthening too).  I agree that kickbox also helps develop and maintain flexibility.  
All of Cathe's kickbox routines are great.  I especially love Cardio Kicks.  Even though I thought of myself as something of a kickbox "purist" b/c I used to practice martial arts, I LOVE all of the non-kickbox moves that Cathe includes in her workouts.  They really expand the range of muscles worked and give the heart a workout.
Best of luck!!
Tracy