While I've done very little with Gentoo, Arch's packaging seemed to me like running OpenBSD's ports tree from cvs, but without the option of an updated-every-six-months super-stable milestone release. It's inherently a "bleeding-edge" distro, which is not necessarily true for OpenBSD. (I run OpenBSD from snapshots/source and update a few times a month, but I also maintain some ports.)

I've only experimented with Arch, though, and I'd also like to hear from others who know both well.