As a fellow gentoo-user, I am extremely happy with it.... until it's kernel upgrade time. I've managed to forget the set of flags needed to make my system work as desired. So it's a pretty painful process.
run `zcat /proc/config.gz > ~/kernelconfig` (i believe that's the right filename for the average Gentoo kernel; haven't booted Gentoo in a long time) and you'll end up with a full list of all the options you chose for your currently running kernel.
This shows all the config options. Options change from kernel to kernel so doing a simple diff does not always reveal the user's intentional config changes. Otherwise gentoo is painless.. in the long term.
Run "make oldconfig" after zcat. Then the previous config changes are applied.
I never used genkernel. When I compile a new kernel version I just eselect it, cd into /usr/src/linux, "zcat /proc/config.gz > .config; make oldconfig" and go through the new options manually. I also have scripts for the modules/kernel installation and the recompilation of packages with kernel modules.