I have used Ubuntu Dapper since the release a few months ago, and i switched with some minor troubles to Debian Etch which i really like more once i had set it up.
A lot is the same, thats logical since Ubuntu is build upon Debian. But there are some differences that make it less userfriendly to setup. I did an install with the Gnome desktop environment included btw.
After installing Etch with the netinstall CD (which works great btw) it configured my xorg.conf wrong. I got a motherboard with an onboard video chip (SIS) and an AGP Nvidia card. xorg.conf was configured for the SIS chip, so the first thing that happened was an X crash (because i'm using the Nvidia card of course). No problem for me, i knew what was going on and how to fix it, but for a 'newbie' it would be the end of his Debian experience - unless hey knows how to use links, or has another OS installed somewhere where he can look up the problem.
Debian doesn't come with a Live CD, Ubuntu does, and imo it is much easier to install an operating system that way, even tho i did not have any troubles with the installation of Debian. Also the 'damn, now i cant do anything' part was annoying me while installing Debian, with Ubuntu i could enter the web with Firefox and read the latest news etc. Now i had to read an old fashioned newspaper during the installation...
The next thing is the documentation of the community. When i first entered Ubuntu i found the Ubuntu Dapper Guide after 5 clicks or something, so setting up things was really easy, especially for a beginner. Otherwise a quick forum search would give me the information i needed. In the meanwhile i don't need it anymore, but i didnt find such a thing for Debian. Same thing for the sources.list. I took quite a wile before i had the repositories i needed to install most of the basic stuff i need. But if you know how to use Google you can find it, but it is less userfriendly imo. And the funny thing is, if you know how it works, it is just so friggin simple (like always)...
Another thing is that when you go look around for packages not in the repositories, you'll mostly find Ubuntu .deb packages, and most (in my case) won't work on Debian by default because of the dependency differences, so you have to go search for those files manually, and if you cant find them, you have to compile the application, which is also not much fun when you are used to do sudo aptitude install <package> or sudo dpkg -i <package>.
There is one thing that bothers me tho, it seems that it is quite hard to install VLC on Etch, on the website of VLC there are 2 repositories for both Sarge and Sid, but not Etch. And the dependency errors i get when i want to install VLC from a .deb package i found aren't much fun, but i found out i'm not the only one with this problem after a quick search with Google. It isn't in the Etch repositories either. But besides this it has everything i used in Ubuntu and it is running nice.
And it installed more stuff i never use then Ubuntu did, but that's probably because i was lazy and let the installer install the complete gnome environment with a lot of applications.
Both Debian and Ubuntu rocks, but Debian needs a little more attention and knowledge to set it up the same way you had to set up Ubuntu. I was excited when i start using Ubuntu, and i'm also excited now i'm using Debian. I feel like i can learn more about Linux from using Debian then using Ubuntu. Ubuntu still runs on another PC here, which my mother uses, and it is running great for 5 months now.