I've been using Ubuntu for several years now. I have it installed on every machine - server, desktop and laptop(s). I have Windoze only on my laptop for the rare occasions that I need it - such as updating my iPod. (I've been burned in the past doing this with Linux and the bad taste still lingers I'm afraid.) One of the things I've really grown to love is the ease with which new packages can be installed - a simple sudo aptitude install <package>.
My first introduction to Linux was Slackware 3, back around '95 or '96. Back then, you generally had to compile packages from source. With the hardware that was available at the time, this could often take a long time. I remember it taking about a day to compile a new kernel. I later switched to Fedora which improved matters with the RPM system. However, I still had to resolve dependencies manually which could be time consuming. I have since tried using yum, but it just seems to lack the simplicity and power of aptitude. Searching for packages seems to take an inordinate amount of time. (I'm perfectly willing to accept that I simply haven't learnt to use it properly, but then I don't particularly like learning new tricks.)
However, I have now decided to switch from Ubuntu to Debian. Here's why:
Canonical have made the decision to drop Gnome from the main release and use Unity instead. Unity was originally designed for netbooks and having tried it out on a netbook, I wasn't terribly impressed with it. I recently tried it again (on the netbook) as I know they've been doing a lot of work on it and I remain unimpressed and switched back to Gnome. I have a 1920x1200 display and Unity would be a complete waste of that space. I realise that there will probably be a way to go back to the Gnome desktop when they do move over to Unity. However, this just smacks of another decision Canonical are pushing down our throats.
Another (somewhat related) issue for me was the decision to switch the title bar buttons from right to left. As a design decision, this is perfectly fine. However, they flatly refused to reconsider or to make it simple to switch them back. (The quote I remember at the time was "This is not a democracy.") Now, it is possible to switch them back, but it is by no means easy - you need to use gconf-editor and edit a particular key which has a somewhat obscure syntax.)
Hibernate & Suspend
I have never been able to get either Hibernate or Suspend to work with Ubuntu. Now, I haven't tries it with Debian yet; so, it may not work there either. But, the numerous times I've tried it (on a fairly wide range of hardware), I've been left feeling that the OS is somewhat unfinished. Usually, the machine doesn't suspend or hibernate and I have to do a cold reboot. In the old days, this was really bad on Linux as disks would often fail to mount if they hadn't been unmounted correctly. These days it is different, of course - filesystem drivers have improved immeasurably over the years - but I still get nervous every time I start a system without having shut it down properly.
I've noticed recently that my main desktop occasionally completely locks up. Sometimes, it really is complete and I can't ssh into it - I have to do a cold reboot. Other times, I can ssh in and shut the machine down cleanly, but not the desktop session. This has been happening more and more recently. In fact, it happened daily (sometimes twice) until I upgraded to an SSD and reinstalled the OS about six months ago. Recently, it's started happening again. With Windows, I figure I can get about two to three years before doing a re-install. With Linux, I would expect to get at least twice that - in reality, it looks like I'm getting about six to nine months.
My main reason for moving is the increasing level of control that Canonical is asserting over Ubuntu and they're seeming lack of concern for their users. I know that Canonical is a business and that the vast majority of their users don;t contribute to their bottom line. They also have a right to do whatever they want with their operating system (within the legal confines of the various licences, of course). I also have the right to choose another OS if theirs isn't suiting me and I've decided to choose one that isn't backed (or controlled) by a corporation and decisions aren't driven by profits. What it really comes down to is choice and I think Debian simply provides far more choice than Ubuntu does.
Finally, I would like to thank the Ubuntu developers for re-introducing me to Linux on the desktop. I had continued to use it on the server, but it fell out of favour with me and I (reluctantly) moved back to Windows. I have to say that thanks to their efforts (and those of the Debian developers), I have almost entirely dropped Windoze from my life.