While there is no question that Ubuntu has brought some high-powered marketing to the table for GNU/Linux, I've heard many discussions recently about Ubuntu actually hurting Linux and the Linux community.
I ask the question, "Is Ubuntu good for Linux?" Of course, this depends largely on your definition of what being "good" for Linux means.
I'll define "good" as having a list of benefits that are larger than the list of detriments. This is not meant to be an exhaustive study of this subject, but merely my observations of the current relationship of Ubuntu to Linux. In particular, there are five main areas in which we can compare the pros and cons of Ubuntu.
ProFor many people, the term "Linux" has a bad connotation. Some have tried a Linux distro in the past, and had trouble with unsupported hardware. Others have heard horror stories about "bricked" computers and loss of precious data due to partitioning problems.
Ubuntu provides a new name and face for Linux. Much like large companies re-brand (ie: change their name) to escape bad P.R., so does Ubuntu provide somewhat of a re-branding. The goal of Ubuntu has been to make Linux trivially easy to install and use, and by consistently supporting this image they've created a brand that's friendly to users, both experienced and novice alike.
ConMany new users give Ubuntu the credit for all of the GNU/Linux software, and hides the hard work countless open source contributors. In a world where credit and bragging rights may be the only reward, Ubuntu may be taking some of the motivation away from open source developers.
My opinionWhile Ubuntu may be getting much of the credit from certain users, it makes up for it by increasing the base of users that can access the software. A larger user base can elevate some developers to celebrity status (eg: "Wow, YOU created GNOME!"). For others, it provides visibility, as their software is exposed to more users and perfected.
ProUbuntu does contribute to the Linux Kernel, and has full-time developers on its staff for this purpose. Even with a small team, they've still managed to fix some major kernel bugs. In addition, Ubuntu is known for including as many hardware drivers as possible, and even supplies proprietary drivers to improve it's own interactions with hardware.
ConUbuntu / Canonical is not as major a contributor to the Linux Kernel as other commercial distributions. It is not in the top 31 kernel developing companies, and a search through the Linux source code shows that only ~100 commits were made by authors with canonical.com or ubuntu.com email addresses. While the value of any contributions can never be under-appreciated by this author, Ubuntu could stand to contribute a little more to the upstream.
ProThis is where Ubuntu really shines. It's not a distro for the latest and greatest features (unless, of course, you want them), but Ubuntu focuses on providing the most up-to-date experience while remaining stable. The 6-month release cycle ensures that, even when the bleeding-edge version of a package isn't included, it will only be about 6-months away.
Ubuntu's motto of "Linux for Human Beings" is a testament to their focus on the user experience. The look and feel of Ubuntu is designed with the explicit intent of making the user experience as easy as possible. They provide graphical administration tools as often as possible for those not comfortable with command line utilities, while still providing Debian-based command line utilities for the gurus.