Apart from creating a lightweight, reasonably secure, custom system and the pride of knowing that you made it all happen, as you go through the book you also get a good picture of what makes a Linux system tick.
Personally, because of the LFS project, my abilities in shell scripting and the Unix command line increased dramatically. This, in turn, led to my being able to contribute back to the project. I initiated the LFS LiveCD subproject, created and introduced to the community a program called jhalfs that automates the LFS building process (it has since been revised by a few talented individuals), and even helped develop the actual LFS book (a copy with some personal changes lives here). It was a fun ride.
LFS continues to receive good reviews as there appears to be many who enjoy the experience of customizing completely their own personal Linux system. Here's a recent article that contains a fairly thorough review. The section on LFS begins on page 3.