1. Run pacman -Syu
2. Pacman failed.
3. Go to Arch website
4. Learn that I shouldn't have run pacman just then, and now I have to follow these steps to un-break my system
The most notable instance was when Arch decided that /usr/lib should be a symlink to /lib. Yeah, barely recovered from that... it was also a reinforcement of my position that basic utilities should be statically linked.
I finally gave up on Arch when a GRUB update broke my system and locked me out of my encrypted root.
Despite the problems, I still find rolling release vastly preferable. If Ubuntu can bring some of the commercial support quality to a rolling release, they'd alleviate a lot of the difficulties that I have with Arch.
That's the number one thing to remember with pacman, _never_ force an update. That is how 90% [citation needed] of problems arise.
I got burnt by the glibc mess last summer, where you had to run a specific set of --force things in a given order to get through and if you didn't, well, you lost your /lib (at which point it's game over, since freaking pacman isn't statically linked).
Response in forums or irc (I don't remember) was "It's rolling release, you had three whole months to update, too bad for you".
Atomic releases avoid the problem because everyone will do this kind of breaking changes at the same point, whereas with rolling releases, you can't force people to update all at the same time, so you'll always end up losing people, once your migration path inevitably becomes invalid because of the way everything goes forward.
Because Ubuntu serves such a different target audience, I expect the whole update process will be more hands-free and possibly a little more smooth. Their mission is "Linux for Human Beings" instead of "Linux for hackers and enthusiasts", so this is a necessity.
I've been using my current Arch install as my main desktop since the great systemd switch late last year, and I just have it do an apper autoupdate on Sundays. I don't even do pacman -Syu, I just check the logs after an update before a restart to make sure it went smoothly.
It kind of won me over, that and yaourt at least. I have Ubuntu on my grandparents machine and sshing into it and using apt starts to feel last century after getting used to pacman.