Rumor has it that I'm a Debian fan. I'll admit it, it's true. If I had my choice of Linux distribution, it would be something Debian-based every time. In fact, I've been using Debian-based distributions for about five years now. I was recently asked why it is that I favour anything Debian-based over the Red Hat's, SuSE's and Mandr[iva|ake]'s of the world. The following was my response:
If you tend to set things up the way you like them and then leave them alone as long as possible, Debian Stable may be a good option. It tends to be released every three years. I use this for servers whenever possible.
If you like to stay reasonably current or are installing on a new machine, I would install Ubuntu, a Debian-derived distribution that releases every six months and patches all releases for 18 months.
If you want to use Linux software as it becomes available, you can use Debian Unstable. In my experience, it is about as stable as Fedora Core or Mandrake. It changes constantly but almost always has the newest version of everything.
Debian's reputation has taken a bit of a hit lately, as the developers often seem to be mired in politics and license debates. The core packages are developed and tested, but Debian itself seems unable to actually release a new stable version in a reasonable and predictable time-frame. For this reason, I find myself mostly using Ubuntu these days.
I am using Ubuntu 5.04 as the base for the next undergrad Linux image at work and am suggesting it to anyone that asks. Ubuntu has managed to release a very easy-to-use distribution with excellent hardware support and a great patching/upgrade strategy. They also tend to stick quite close to Debian's goals, though they do allow a few extra pieces of software through, such as Nvidia's binary drivers and Macromedia's Flash player.
Once nice thing about sticking to Debian-based distributions is that even if development slows for a particular distribution, you can always move to pure Debian. For me, this means that if Ubnuntu goes out of business, I can just make one small change and then I'm back to using normal Debian.
I've already used this at Queen's. When I started here I suggested that people use Mepis - another Debian-based distribution. While Mepis is great, I've never been comfortable with the fact that they pin against Debian Testing. Because of this, I have since moved the systems with Mepis installed over to Ubuntu's packages. For the most part, the transition has been seamless.
As for the nuts-and-bolts differences, I find that Debian-based distributions tend to have better dependency resolution, and they insist on putting files in slightly different places. Other than that, it's just Linux as usual.