Benefits:
I have spent too much time arguing against the perceived deficiencies of systemd (such as: "it is not written in bash", "it was started by Lennart Poettering", "I don't like the (optional) on-disk format of the journal", "it uses dbus", "systemd's PID1 does more and is bigger than sysvinit's PID1", "I think there might be this other project that possibly is doing something similar. I don't really know anything about it, but I'm pretty sure it is better than systemd" and I'm sure there are many more). I strongly believe that 1) all of these perceived deficiencies are not deficiencies, but are actually benefits 2) even if I'm wrong, these things are not hugely important. So, with that out of the way: let's ignore all of those old boring arguments and I'll outline a few things that I find awesome about systemd, and why I think we should all be very excited about soon being able to use it. In no particular order:
0) it is hotplug capable: systemd assumes that all resources may appear and dissapear at any time. If you plug in your external harddrive after systemd has booted, it will be fsck'ed and mounted correctly. This is unlike initscripts which relies on all disks being enumerated and ready when it starts fsck, and then it relies on fsck of all disks being finished before it starts mounting any of them. Hotplug is important, not only because it is convenient to be able to insert/remove hardware while the system is running, but also because that's how the linux kernel does boot: every device appears to be "hotplugged" as the kernel becomes aware of it, so with a very fast boot we can no longer assume that all devices are ready and waiting for us when we need them (even if they were plugged in when the computer started). In reality this is often not a problem, but if you ever had your rootfs on an external USB harddrive you might have experienced problems (and as things become faster and faster more problems like this will crop up).
1) we can know the state of the system: systemd keeps track of all daemons, and all processes that are started, and who owns what, and when something fails, etc. Also, using the (awesome) journal all syslog() entries and writes to stdout/stderr by all processes are captured by systemd. These are stored with enough meta-data so that you can very easily retrieve say "all entries from a certain service/binary/pid" or "all entries written by the kernel regarding a given device", etc. In addition to logging this information, and showing it to you, systemd will allow you to specify (easily) what to do in a wide range of possible error-scenarios: "a service shuts down normally/with an error/ on a signa" or "a service has not sent its watchdog signal in the designated time" or "a service has shutdown with an error 10 times in the last half hour". The recent addition of hardware watchdog support also allows you to say "restart the machine if systemd itself is not responding".
2) it is modular: all of what is now rc.sysinit is split out into many independent services, each of which is well documented and easy to understand. I.e., if you don't like how systemd e.g. does it's fstab handling, then you can write your own little helper (in bash if you wish) to replace the official one. Doing this in the old initscripts is much harder because 1) it is not so clear which parts of rc.sysinit are dependent on eachother 2) any changes you do you'll have to merge on every update.
3) it allows dbus/udev to go back to doing the task they are meant to do: both udev and dbus are currently (mis)-used to start daemons/long-running services on demand. In the case of dbus this is by design, but in the case of udev it is not. Either way, it is not what those daemons were built to do, so in keeping with the UNIX principle of one task per daemon, it is great that we can now let systemd (whose job it is to manage daemons) take this over. That is, udev and dbus can both signal systemd to start a certain daemon, and it will behave like if it was started in any other way (you have the logs, status etc). One problem that this solves is the inherent race-condition in some daemons (I think bluetoothd was guilty of this at some point) allowing both being started as soon as possible on boot (say by putting it in DAEMONS), and to be started on-demand by dbus. Systemd makes sure that both these things can happen, and if they do happen at the same time you will only end up with one instance of the daemon as expected.
4) we can reduce the number of explicit ordering dependencies between daemons (this might require changes to the daemons in question): using socket activation, automounting and dbus activation we are able to forget about stuff like "dbus must be running before we start avahi" or "yp-bind must be started after networkmanager". systemd can create sockets early on, before any daemons are started, and then pass the socket to the relevant daemon when it gets around to starting it. This means that anything can connect to anything else, without caring about whether or not the other daemon has finished starting yet. The kernel will simply buffer the requests whilst we wait and deliver them when it can. Notice that this does not actually remove any dependencies (so if there are circular deps, they will still be there), but it means we no longer have to specify them, nor worry about races between them.
5) we get a lot of security/sandboxing features for free: you can simply add some configuration options to the unit files in question to isolate them from the system in various ways. This was of course also possible before, but it required you to write lots of boilerplate code in every rc script, and this kind of things are very error prone, and best reviewed by people who know about this stuff.
6) systemd service files can (and hopefully will!) be written and distributed upstream: rather than every distro writing their own rc script (with their own set of trivial bugs and misunderstandings) the people who know the software the best (upstream) possibly with some input from the people who know the init system the best (systemd devs) can write "perfect" services that should just work everywhere. We have seen some of this already, and I think it is hugely benefitial. Even for distros who don't pick up systemd yet, this will allow them to at least have an idea oh how the software is meant to be initialized.
7) systemd is a cross-distro project: every major and many, many minor distros have had people contributing to systemd. last i heard even two debian devs have commit access to the repo, among many others. systemd upstream is very accommodating of different needs and different use-cases (as long as they are presented on technical grounds) and have been a pleasure to work with so far. We are getting the joint experience of a lot of people/projects who have worked on different init systems for a long time, I think this is one of the most important "features" one could have.
logind will finally deliver on what consolekit was supposed to do: we can now track user sessions and seats, assign permissions to resources on-the-fly etc. This should be the topic of a separate message, as it is too much to get into. Suffice it to say that I'm very happy that we are finally getting these features working as they should.
9) systemd is fast: Or is it really? Some claim there is no difference compared with initscripts, a very few claim it is slower, the vast majority claim it is a lot faster. I claim that I really don't care. The above points (which is just what I could think of at the top of my head, so not exhaustive by any means) would hopefully convince you that systemd is the right choice irrespective of its speed, and once you try it out you might be in for a pleasant surprise
