I don't know how "good" they are as examples of abstract art in general, in my opinion. I'm a big Paul Klee and Kandinsky fan for that in terms of getting started on that path, but I do like Twombly for his use of paint, layering, scratching, and composition. I think it is interesting, playful and childlike. I like the little scribbling he does that is not recognizable. It's his own made-up language, like a child might write (this is all my own observations by the way, not one of any kind of professional person or critic), or yes, perhaps the graffiti of some ancient civilization. I like the space he uses between shapes and the scribbles where things begin to breathe. Perhaps the scale of the work has something to do with it. There's a lot to notice on the canvas when looking at it all in person. Many different textures, which I love. The chalkboard-looking ones don't have the texture so much, but I love their simplicity and minimalistic qualities. To each their own.