What a masterpiece. No matter how many times you watch it, there's always something to be gained. It's not one of those "sit back and be entertained" shows. You actually have to put some thought into it to understand it. Even so, you probably won't get everything, and it raises more questions than it answers.
The Signature Series box set is one of the best anime purchases I've made since I started two years ago. Always worth another viewing.