I'm still technically an Arch newbie having only been using it for 2-2.5 years. Gentoo was my primary for a number of years prior, so I've never felt Arch was overly flaky--leastwise not in comparison with the extraordinary breakage in Gentoo that can occur if you're not careful (but hey, it happens). So, I think it's important to keep in mind that the "feeling" of instability and flakiness is probably taken relative to some benchmark or experience that is difficult to divine from a few short sentences. In my experience, Arch has been quite stable and usable. It's not without its warts, of course, but I think it's one of the drawbacks you have to live with if you're using a rolling release distribution. (I don't know how Aptosid seems relative to this since I only use it on a laptop infrequently, but I'd imagine it's not impervious to odd breakage either.)

> Miscellaneous bugs which force me to boot from a USB and chroot to reinstall packages and/or reboot.

This one's a bit of a nuisance, but I seem to have only had it occur when mkinitcpio runs during an upgrade. I don't know if this was due to a bug in earlier versions, but I've since made it a habit to run mkinitcpio immediately after upgrading. I think that's culled about 98% of the mysterious breakages for me, usually related to the kernel upgrade process (or modules missing from initrd). Otherwise, it's been fire and forget.

Forgetting to merge new/updated configs is also a slight nuisance and can lead to breakage. This is partly why I like `yaourt -C`, but it doesn't always catch everything.

> The systemd migration was not very clean. I'm glad Arch switched to systemd, but the migration was tricky, and it also came within a few months of another rather tricky update (/bin) that broke things for a lot of people.

I had this experience, too, but in my recollection, it was mostly due to the fact that Arch was 1) an early adopter of systemd and 2) the full migration took effect during a time when there were unit files only for the most common services. Fortunately, nearly everything else I needed had been added to the AUR. One of the things I like most about systemd is the relatively low bar of entry to create new unit files for services that didn't exist. But, I will concede that some of the early helper unit files were a bit buggy, and the near-constant changes caused an unfortunate bit of havoc. I think that was largely fixed when netcfg was dumped for netctl. netcfg was a pain in the rear.

Which reminds me: If there's one thing I really missed coming from Gentoo to Arch, it was Gentoo's idea of configuring network devices. Until netctl, the only way to include some of the more unusual configurations in Arch required some magic (and lots of tweaking) with the netcfg scripts in order to get the right arguments passed to `ip` without netcfg going insane. Sigh. netctl is much more forgiving and better behaved.

> I'm running Wheezy on another machine, and shellshock still hasn't been fixed there, whereas Arch had it patched within hours.

Yeah, that was nice. Although I will point out that while the initial shellshock vulnerability was fixed almost immediately, the remaining vulnerabilities in bash weren't patched for a couple of days (still better than most everyone else). The final patch was rolled out about a week later when some of the "unofficial" patches were coming out from Redhat that actually did fix bash. Though, I gather from mailing lists linked from here that the disillusionment with upstream maintainers, confusion, and the 3rd party patches/patch sets/collections probably didn't help. That was messy.