I love kpc, don't like tae bo, haven't tried p90x. Lately I go to a fitness kbox class with heavy bags, so I have not bought more cathe kickbox dvds.
Here are some others to look into:
I really love Kimberly Spreen's Absolute Kickbox. Great music, fun routines, and she has this great weighted bar section.
I'm torn on the Amy Bento dvds, I want to like them, and sometimes I do, sometimes I don't, she has a few, I think you can get clips at collagevideo.com. I can't keep up with the choreography on the one called Kickbox Xtreme. But the In the Ring dvd is a nice one to work in sometimes.
I love the Ilaria Montagnani power strike, it's an awesome workout to work on your form, and just a great overall energizing workout. She has a new one coming out very soon, the one I have they sell on Amazon and I think advancedworkouts.com or collage.
I do not like Tracey Staehle strikezone at all. Did it once, will not do it again. But others like it.
Oh and of course, Turbo Jam, which is kickbox with a dancier vibe, but I found after one run through it was easy to follow and the music is fun.
HTH