Back in 2003 I became a gentoo developer. I had been using gentoo prior to that as my Linux distro since it had good amd64 hardware support pretty much out of the gate. I had pieced together an amd64 box and at the time I thought trying out a new Linux distro was a good idea.
Then, I worked really hard on getting ppc64 up and running. At that time, while you could run 64 bit kernels on Power and ppc64 hardware, the user space was pretty much all 32 bit.
Gentoo today in 2014 is still in my opinion a good distro. There are essentially two modes of operation where you either build a package at the time you install it, or you can install from binaries via http://www.sabayon.org/.
As an open source developer I treasure the ability to easily install and test anything from source. Further I very much enjoy the ability to change compilation options for fiddling -O3, -mtune etc options to test out new compilers and see how performance improvements in codegen is coming along. I find it a much better environment than Open Embedded.
For me, I’ve been adding arm64 support to gentoo and this will be my primary focus in my “copious spare time.”