I've used Ubuntu 12.04 as my primary OS for these past 8 or so months. It's great. Stable, productive, powerful, modifiable stock, it also serves as the foundation for many other distributions. The only other OS I really use is Linux Mint 13, which is built on Ubuntu.
Of the Ubuntu derivatives, I've tried mainly. Just now, I loaded up live sessions of Elementary OS Luna beta 1, Pear OS 6.1, and Bodhi 2.2.
I'd read good things about Elementary and Pear, and was revisiting Bodhi. I was thinking of replacing Mint with one of the former two. Cinnamon is good, but never really hit the sweet spot for me.
All three were good in their own way. Pear does an admirable job imitating Mac OS. The developers achieve their goal, but the effect was sickening to me. The big silver pear-with-bite-missing seemed crass. I changed the wallpaper. The OS was good. Well-designed, with a speedy implementation of Gnome 3, I understand why it gets high reviews. Elementary was similar, though, as it's a release-when-ready OS, it had a feeling of greater attention to detail.
Bodhi was much improved, but E17 still looks strange. As one reviewer put it, E17 looks dated and, in trying to look slick and up-to-date, draws even more attention to its dated graphics. Again, though, the OS succeeds at what it's trying to do: provide a minimalist, easily-modifiable, and attractive base.
That said, none of them measure quit up to Mint 13, which I will be keeping. Cinnamon in 13, despite being a little ho-hum, has the feeling of a mature product. Something stable and helpful to productivity, or play, that will get out of the way of the user. Pear, Elementary, and Bodhi all feature unique desktop environments, but ones that still seem idiosyncratic upon user interaction.
The developers of all three are obviously talented, and design features are elegantly implemented. However, it just seems like Cinnamon has "been through" more. More revisions, more testing, more eyes. I'm pretty sure it literally has, but that's also how it feels.
So, I'm sticking with Mint 13 Cinnamon due to a range of reasons I will vaguely call "greater maturity".
Mature distributions in Linux are not very many, despite the 600+ unique listings on Distrowatch. (The BSDs are quite mature, I suppose, for their type of work).
We have the major distributions that serve as foundations for others:
Debian, Ubuntu, Arch, Slackware, openSUSE, Fedora, Gentoo, Red Hat.
Of these, Debian, openSUSE, Fedora, RH, Ubuntu, and Slackware are full-featured OSs out of the box. Arch and Gentoo are powerful, but rely on the user to, in effect, be the architect of their own system. The user then has to devote the requisite man hours necessary to be one's own sysadmin as well as get other stuff done.
In addition to the previously mentioned six, there are a number of derivatives, few of which could be called fully mature in the way their bases are. (I'm excluding minimalist, niche, specialist OSs from this discussion).
Xubuntu, Kubuntu, and Lubuntu are not really forks. The same goes for KDE versions of Fedora and Gnome versions of openSUSE.
Which leaves, really, Mint, and perhaps Zorin. PCLinuxOS and Mageia are not derived from the aforementioned six and are excellent distributions in their own ancestral tree.
There are many, many good and very good OSs like Pear that fall just short of having the collective experience and user backing of Mint and the like. #!, for example, Semplice, Pinguy.
It doesn't take too long to notice a trend in the open source community. Projects take a long time to reach maturity, and developers love to fork and start new projects. So, we have many excellent projects that slowly lose inertia over time.
One OS that stands out for quality is Fuduntu. Perhaps because it runs on Gnome 2, or perhaps because Fewt, it's project head, is a very experience developer, it has the feeling of a very mature and stable product. But, even then, it suffers from a lack of resources. Upgrading software can often lag because it doesn't have its own repository servers.
True operating system maturity, in which one can easily set up a system, leave it alone, and do work with smooth worfklow, is surprisingly rare among the forest of options we have among Linux distributions. There are maybe ten distributions, not counting respins, that accomplish this and, therefore, can contend with operating systems like Windows 7 and Mac OS X.