Hi, I just registered but I have been Siduction for some months. Other distros that I use on a regular basis are Arch (systemd), LinuxBBQ (which is based on Siduction of course. Some versions have systemd as init), Mint (upstart? I don't know to be honest) and Crunchbang (debian stable, sysV I suppose).
My experience is that Arch with systemd boots fast and closes very fast. LinuxBBQ (siduction+systemd) is also fast, slightly less - perhaps because Debian can't use the newest systemd versions? I use lightweight Windowmanagers, such as Openbox and i3wm. I haven't done scientific comparisons, but it feels like the more systemd, the faster boot and close is.
Stability wise, I can't tell differences. I did have X11 crashes in Arch. I haven't really dived into it, but I suppose this is a conflict of the kernel with my hardware (Lenovo Thinkpad), I doubt that systemd is to blame, but you never know of course.
The concept of systemd appeals to me. Yes, it does look monolithic and non-UNIX like, but to me the explanation of Lennart P. is convincing: the alternative is to have lots of seperate config files. I would welcome some unification, one way to set keyboard, mouse, time, locales etc. I a windowmanager most people like it when there is only one file to edit for settings.... so, why not have one system for init and lots of "householding" (keyboard setting, screen settings etc.) as well? I must admit that most of the technical discussions are WAY over my head, I'm just a simple user :-)
Anyway: keep up the great work, siduction crew !!