I get the impression that most people are migrating the other direction, from Debian to Ubuntu. Or actually, from lots of other Linux distributions to Ubuntu (but Debian is significant because it's Ubuntu's parent distribution).
I have only good things to say about Ubuntu -- it's the distribution that allowed me to switch to Linux. It's the first distribution to correctly configure my wireless card during installation (a task that both Windows XP and Vista fail at), which allowed me to get my questions answered online. I started using Ubuntu Dapper Drake, and have since installed each new release. I even convinced my Mom to switch to Ubuntu from Windows. However, since installing Debian Unstable (Sid) a few months ago, I've come to prefer Debian.
Ubuntu is still a relatively young distribution, which, combined with the aggressive new release schedule (every six months), means that it's changing very quickly. In general, I think this is a good thing. Each release becomes increasingly user friendly. But, it turns out that I don't really need a distribution that is that user friendly. I'm a pretty quick learner, and I've gotten used to editing configuration text files instead of clicking buttons in an Options menu. I think I became acclimated to how a Linux system works, and Ubuntu in particular, around the Fiesty Fawn release (maybe it was Gutsy Gibbon). Ubuntu does a fair amount of hand-holding for inexperienced Linux users, but once I got enough experience, I found that I could work more quickly and more precisely without many of the GUI tools. As I installed newer releases, I found myself disabling many of the new features. As I was setting up and customizing the new Hardy Heron release, I noticed that the xorg.conf configuration file was no longer in use, and I no longer knew how to set things up the way I liked them. (The xorg.conf file may be an odd thing to miss, as it is often the first truly confusing thing a new user comes across). So, hoping to find something more like I was used to (Fiesty Fawn), I tried Debian.
Debian was exactly what I was looking for. It had the same underlying system I learned on (specifically the APT system, which is awesome), but in less-shiny packaging. I would describe it as a more transparent distribution than Ubuntu. Debian doesn't hide as many of the available options (which is often the root of complexity) as Ubuntu, and consequently it's probably not as user-friendly. While maybe daunting to a new user, it was refreshing for me.
I first installed Debian Stable (Etch) (stable sounds good, right? at least it sounds better than unstable). Debian Stable is just that. The trade-off, though, is that the software is not very cutting-edge. Since I was already spoiled by always having new releases of Ubuntu, I knew I would have to upgrade to Unstable or go back to Ubuntu. During the upgrade process, I discovered that Ubuntu's popularity is very nice when it comes to finding answers online. It's not quite as easy to find the answer to a Debian question, but I survived. Unstable seems to run software that's just as new as the new Ubuntu release, if not newer. And, despite its name, I have had no problems at all with instability -- everything's worked great. So now I have a system that works in similar ways as my old Ubuntu system, with newer or just-as-new software, but that preserves a bit more of the classic linux structure. I'm pleased.
Another benefit of Debian is that it seems easier to have a minimalist installation. I felt like Ubuntu was getting slower with each new release, perhaps because it increasingly attempted to be everything to everyone. That definitely has its place, and I can see how it's helpful for Windows converts, but it's not for me. Also, although I'll miss the excitement of a new Ubuntu release every six months, I might actually get a bit more done if I'm not installing a new release twice a year.