I'm not GP, but I read and worked through almost the entirety of Volume 1. I found it both approachable and enlightening. I have read (but not worked through) large chunks of Volumes 2 and 3, I have not purchased a copy of Volume 4 yet.

Volume 1 really works well if you also work through Concrete Mathematics (my professor and I started on Volume 1, diverted to CM, then back to Volume 1). CM provides enough additional math to get through chapter 1, where I was having a bit of trouble on some of the math heavy portions.