At a certain point in my life, I've decided that it was time to throw away Windows definitely. Linux was sufficiently mature, there was sufficiently good office productivity tools, which was the biggest concern to me at that point. I'm user of Unix systems since 1986, including several Linux distributions since 1994. I've banned Windows from my life in 2004 and I'm using Linux all the time on all computers I own ever since. Apparently to me, Ubuntu was the best option for a Linux distribution at that point, once it had recent good enough versions of office productivity tools, browsers and mail readers.
I remember that, in 2005, I did a system upgrade. Well, after that I was not able to connect to the Internet anymore. Pretty bad! After spending a couple of days researching what the problem was (thanks God I had a laptop at hand!), I've finally discovered that my network card was not supported by the Linux kernel anymore. Actually, it was a bug, a regression. After evaluating available options and effort involved for each option, I've decided that I should simply buy and install a supported network card.
A couple of months later and another system upgrade. This time the printer stopped working. Long story short, two more days wasted trying to find a way to circumvent the difficulty. I don't remember anymore what the solution was, but I remember that my wife complained a lot because the printer was not working anymore.
I certainly recognize the importance of Ubuntu for popularity of Linux. Thanks to Canonical, Linux gained a lot of momentum and became a relatively popular operating system to non-initiated end-users. Before, Linux was only intended to nerds.
Because Ubuntu evolves quickly, many times I've installed it again, in particular on laptops which I can quickly reinstall everything if needed, without taking the risk of losing important stuff.
But I had bitter experiences. Not only those two I've described above, but several others. The "upgrade-and-break-stuff" thing happened many, many times. The last time was just two months ago, when I was playing with XBMC on a laptop, taking TV feed from my workstation: after the upgrade, it became impossible to watch TV again over the network.
I definitely prefer Debian. I use it on production systems. Debian is rock-solid stable, secure and reliable. In 7 years I'm using Debian everyday, I've never had a single problem of regression. I can blindly upgrade packages or even the entire version of Debian without any trouble: Debian simply works.
Update: I've got rid of Ubuntu, definitely. I'm now using Debian Wheezy and compiling from sources some tools I need in order to go ahead with software development work on GPUs.