Despite Wubi's lack of a focused effort to consider the knowledge-level of the intended audience, the results are worth waiting for. Although the Ubuntu CD menu warns that, when installing inside windows, "disk performance is slightly reduced," any loss of performance is minimal, amounting to a few percent at the very most. The resulting Ubuntu installation cannot go into hibernation, and its bare desktop could do with a few icons to help guide the curious, but otherwise, you could easily forget that you are running from Windows.
Uninstalling the Wubi-installed version of Ubuntu is also straightforward. Its files are installed in a single directory in Windows, and you can remove it the same way as any other program -- by clicking Add or Remove programs in Setup. The removal is thorough and clean, although unsophisticated users might wonder why Windows gives them the message that "Wubi has been uninstalled from your computer" when they believed that they were removing Ubuntu.
Wubi is designed as though its audience were GNU/Linux users of intermediate experience. It needs an an overhaul of the interface -- in particular, with more explanation and consistency in the use of names (probably, "Wubi" should be replaced everywhere with "Ubuntu). With these changes, though, it should be a useful addition to the Ubuntu repertoire. Probably, as with the live CD install, it will encourage other distributions to offer the same functionality within the next six months, starting with the other Debian-derived distributions that can use Wubi without major modifications, then extending to those with other package systems a month or two later.
But will Wubi succeed in its goal of encouraging Windows users to take Ubuntu for a test run? That is hard to answer. On the one hand, a Wubi-installed version of Ubuntu gives users a better sense of its speed than a version on a live CD does. Unlike a live CD, it also offers users a chance to explore aspects of a GNU/Linux system such as package installation.
Yet, on the other hand, unsophisticated users might be more comfortable with a live CD than in making changes -- even temporary ones -- to their operating system setup. And although Ubuntu emphasizes that a Wubi installation requires no partitioning, that fact is beside the point, because obviously neither does a live CD. Moreover, a Wubi install takes at least as long as a CD install that includes repartioning, so unless you have a deep-rooted fear of partitioning in general -- which some Windows users may very well have -- it has no advantage over a dual-boot installation.
Still, free software is all about choice. What matters, really, is that Wubi gives non-GNU/Linux users an additional way to satisfy their curiosity about the operating system. With any luck, the more choices that are available, the more Windows users will actually act on that curiosity.