If you only think window art is 'kinda neat', you probably don't need this refill set. There are a few more colors -- the white and clear make some really pretty Tiffany glass style effects possible, for those interested in spending the extra time -- but really there isn't that much more here than you'll find with your standard set.
I do have a couple issues with the refill. My primary one is that there just isn't enough black! I'm always running out! Also, I've noticed the window art tends to fade in the sun -- to be expected, I suppose -- and also doesn't store very well. Even when stored as recommended by the book, my window art got a little flat and squished looking.
But overall, this set is a lot of fun :) I have my windows decorated with butterflies and tulips, and my shower door decorated with fish. It's an easy, fun way to bring a little bit of color into my house.
My 2 children love the Klutz books. The window painting book is their favorite and this refill kit is great! It comes with several colors and the big bottles of black that you need for the outlining. My daughter loved the glittery paints. The kid went through the paints that came with the book in just a few days - their windows are covered with clings, and this refill kit was just what we needed.