I would recommend Ubuntu to everyone starting out with Linux, and even to many intermediate and advanced users.
For new users, Ubuntu offers a great way to familiarize yourself with GNU/Linux distributions. There is a point-and-click installation with few technical questions, the system comes with a sane choice of default applications (one application per task) and most of the configuration is either done automatically, or available through graphical utilities. Ubuntu has probably the largest user base among Linux distributions as of today, so any issues you might experience will probably be solved very quickly if you mention them at e.g. the ubuntu forums.
For the intermediate and advanced users, the system should not "get in the way" since you can still configure the system manually through files in /etc and handle package management through apt-get and apt-cache. There is also an extensive number of binary packages available in the default repositories, which should cover everything from your favorite applications to support for obscure programming languages.
If I have to mention something negative with Ubuntu, they would mainly concern advanced users. The first thing is that the system comes in releases twice a year (compared to rolling release distributions), and upgrading between these releases can sometimes be a hassle. The second is that if you want to compile a lot of software from source, it might start getting in the way. You have to get used to installing extra "-dev"-packages before compilation, since the regular library packages don't contain the header files needed during compilation. Another thing is that the only easy way to create your own packages is to use "checkinstall" - which is a great tool, but some people might find ABS, Portage, etc more flexible. Finally, minimalistic users might prefer using a system that installs only the bare minimum and lets you build from there - although you could argue the server-edition of Ubuntu should fit this purpose.
I would rate this distribution "excellent" as an operative system for the desktop. I recommend it whether you're a new user that wants to try Linux for the first time, or an advanced user that wants something that "just works".