Before Xbox PIX, graphics debugging involved thinking really hard about a huge state machine that lives in a separate, uninspectable memory space and is modified in hundreds of locations in your program. After PIX, graphics debugging became easier and more fun than any other system I've seen. The operation of the whole machine is laid out visually in a scrubbable timeline. All of your assumptions are easily testable. Its heaven, I tell ya.
If ApiTrace can get close to the functionality of PIX in a cross-platform, cross-API, open-source way... Its a big deal for me, at least. :)