Well, Arch Linux finally pulled the plug on its old Init. About time, I guess - but I'm a procrastinator, and even though I was generally aware of systemd I had no compelling reason to switch until I was forced to.
So, now I'm forced to!
The Big Deal about this for Arch is that /etc/rc.conf is going to basically stop working for most things - you need to take any deprecated stuff out of there and put it where it now belongs.
Here's what you'll probably need to do to make the change in Arch when you lose your old init scripts:
1) Write your fqdn to /etc/hostname (nothing else - just your fqdn and a carriage return)
2) If you don't use UTC for your system clock (e.g. dual boot Windows), be sure /etc/adjtime reflects that fact (timedatectl does this)
3) Make sure any modules you need are loaded from /etc/modules-load.d/{whatever}.conf - one module per line, nothing else
4) Get rid of any legacy services. The easiest way for me to do this was to actually leave the DAEMONS line in rc.conf and reboot with systemd enabled. From there, you can figure out what's coming from rc.conf with: 'systemctl | grep -i legacy'. If you've got anything left, use 'systemctl enable {thing}' to enable the systemd version, and remove it from rc.conf (some specific things I remember - syslog, cron, ssh, iptables)
systemd is pretty sweet - like SMF from Solaris. My SSD-based system hits gdm like... nearly instantly with it enabled. But it'll take some getting used to for sure.
This is one of those pain points I'm glad Arch forces upon me, and systemd represents a real improvement in how init works. But I'd never have bothered with it voluntarily :)