Debian Sid is the original rolling release. Its advantage is is that you are on a rolling-release distro but it's still Debian, so all the system knowledge you accrue will apply seamlessly to production Debian servers. Most rolling-release distros, on the other hand, are typically not used on production servers.
Sid is not a rolling release. This is a common misconception.
In preparation for a new stable release, it goes through what is effectively a package freeze. Maintainers are discouraged from updating packages in Sid in order to focus on the new release. So almost no new updates are uploaded to the repositories until Stable is finally out.
That's not exactly a hard rule, just stuff maintainers do because it is far easier to still have normal flow of unstable -> testing for the freeze period, and keeping 2 ("actual unstable" + "unstable on the way to the new release") would be a big burden for maintainers.
Also, packages are tested in sid/unstable and migrates to testing which then becomes the new stable when it's ready.
And packages are tested across releases, enabling one to dist-upgrade to a new version of the system easily. I think it's piuparts that does this job of testing packages among themselves too.
Like toyg says, I really enjoyed having a familiar working environment both on servers and on my workstation. When big changes would come around (like the transition from SysV init to systemd), I'd start getting hands-on experience early, and upgrade production with more confidence.
Between testing/sid/experimental you also got the choice of running "somewhat stable" vs "bleeding edge"; I'd usually stick to testing (except around the freeze leading up to a release).
I rather meant irregular with regards to Debian Sid not Debian in general.
Arch only has rolling release mode so everyone is using the latest (thus any potential issue in Arch will affect bigger % of Arch users) while Debian has stable and testing and I was wondering if that'd make Sid less stable for practical use (since "it's not stable").
I don't want to start a distro war here, just wondering about some anecdata of actual distro users.
I'd think that both distros work with upstream as much as possible when bugs are found. I'm too lazy to do it, but you could check both distro's bugtrackers for activity to know if one really generates more checking than the other, but I'd be surprised if in practice there's a big difference. Maybe pick a few packages, see how fast fixes are merged?
Anecdata, if you aren't advanced user with new hw, Sid might be more robust, otherwise Arch is superior in almost any way. I wish it wasn't, I like Debian community more, it is less elitist.