Linux may never be a factor on the desktop, but I think Ubuntu has a good chance.
Wait. Isn't Ubuntu just a Linux distro? Well, that depends on how you look at it.
Is FreeBSD a factor on the desktop? OS X is based on FreeBSD, but no one thinks of FreeBSD as "a competitor to windows", or "a user friendly OS".
Of course, Ubuntu to Linux is not really like OS X is to FreeBSD. Ubuntu didn't invent a new desktop, it's based on Debian and it uses GNOME (or Xfce or KDE). But there's still a point to make here.
While the open source community as a whole might fail at designing a great desktop experience, this limitation doesn't necessarily apply to Ubuntu. The design efforts there are lead by Canonical, and it's not horribly fragmented. It has so far produced some pretty decent results. See https://wiki.ubuntu.com/SoftwareCenter and https://wiki.ubuntu.com/NotifyOSD as examples.
The desktop as a whole is not quite there yet, but it does stand a good chance, and it's already making a lot of strides.
On my laptops, I've used Ubuntu since Badger (2005). I went to Ubuntu from Debian, because everything on the laptop suddenly worked. No more kernel compiles or weirdy-beardy deep hackery.
Linux on the desktop has arrived, I thought. But I'll wait one more iteration before recommending it to my clients.
Dapper (2006) came and broke lots of things. Months later, all is well, mostly.
Once bitten, I didn't upgrade until Heron (2008) was released, and when I upgraded I did so one version behind, to Gutsy (2007); thinking that it should be stable.
Many things worked, but some, like video drivers and wireless networking, didn't. They'd been working fine since Badger.
By Jaunty (start 2009) everything was working. I still have this machine on Jaunty and no way am I upgrading it. It has some weird video behaviours, but Ubuntu has never bedded these down; it's always a bit of a lottery.
2010, I bought a new laptop (Sony Vaio). I loaded Lucid (2010 Long term support version) and I can't tell you issues it has. There is a dedicated group for resolving the issues. When I plug in the USB soundcard I use on the aforementioned machine -- it's playing now -- a card that has worked since Badger, the machine locks solid; it needs a power-down to restart it.
This is not an uncommmon story when dealing with Ubuntu. This and the vagaries of Shuttleworth. As he says, "This is not a democracy".
Ubuntu, like all things, is great when it works, but is constant shifting sand under your feet. And stability -- on the same machine, same OS -- is fundamental, imo. Ubuntu doesn't have this, and history has shown me that it is unlikely too. I've chosen to move away because of this.