Debian has historically run on an approximately 2-year release cycle (and more recent releases have been closer to that cadence). Right now, Testing is kind of old, because it's been frozen in the run-up to the Wheezy release. In a month or two, Testing will probably be mostly current, and will stay that way until about 18 months' time when the Jessie freeze kicks in, and then it'll be a slow wait until the release of Jessie and then the cycle repeats.

Ubuntu is too... opinionated for my tastes, and other distros are too conservative (CentOS/RHEL) or too wild (Arch, Gentoo). Debian Testing seems to have a high-quality, comprehensive package archive and keep reasonably up-to-date, and if the price to pay is that for six months every two years you get a stable desktop environment instead... no complaints from me.

EDIT: The usual lag between Testing and Unstable is 10 days, except in big, complicated transitions and dependencies (say, when a commonly used shared library is being upgraded to a new, incompatible ABI). Again, I could probably be more up-to-date if I tracked Unstable instead of Testing, but I'd rather wait for Debian to nail down its migration plan in Unstable so I can upgrade smoothly, rather than have the latest and greatest and have to rebuild my own packages occasionally.

I have used unstable as my main desktop for over a decade now.[1] At first there were occasional issues but these went away as I became more experienced with debian. It has been years since I encountered a bug that caused me to lose more than an hour of productivity. The most recent issue that I have had with unstable was over a year ago and it was simply a matter of pinning a version of libcairo. The only reason I remember this so well is that someone else on HN was having trouble and my comment[2] on how to fix it was my highest rated comment at the time.

[1] To be honest I actually run a mixed system with a few packages (not just oneoffs but big things like xmonad/iceweasel/awesome/git/pandoc/git-annex/gitit) from experimental. If it has been a long time (5-7 years) since you tried unstable you can think of experimental as what unstable used to be.

[2] https://news.ycombinator.com/item?id=3907796

Once, this was a valid comparison (Ubuntu/Sid). Now Ubuntu is a strange beast, a world apart from other distros in goals and just in the basic desktop experience.

aptosid is probably a better choice if you want a more stable Sid these days without the Ubuntu kaleidoscope.