From my experience in seeing people use Ubuntu, I have thought up this graph to show how they have found it. In general I find that people actually like it, and except for when it breaks, they like it more than Windows (unless they are gamers, in which case they will always like Windows). I will discuss the reasons for this later, but first, consider this graph. The horizontal axis is the computer-"savviness" of the user, from grandma to guru. The vertical axis is how much the OS would suit them.
As expected with any version of Linux, the amount it will suit them increases with their savviness. This is also the case with Windows, however it is generally lower. Although this may be a subjective point of view, this is the average user: they don't touch anything on their computer settings-wise, they don't want to install software, they just want to use the computer. These are things which will generally not break Ubuntu, and Ubuntu has this nice thing called permissions that keep them from accidentally borking their install. XP doesn't have this feature, so if somebody accidentally deletes a bunch of things they don't recognize, there goes their system. Vista is not (as far as I can tell) like this, instead it locks them down like they're in prison or something which is probably less user friendly. The reason Ubuntu is higher than Windows on this part is because Ubuntu does not have malware and Ubuntu will not randomly corrupt itself (at least not as far as I have seen). In my 5 years of using XP, these are things that usually happen on a regular basis. When a non-computer-savvy person comes across one of these occurrences, they panic. Bad!
We now see that there is a big spike for Windows and a big drop for Ubuntu partway across the graph. This represents people who think they know what they are doing and want to learn/fix everything, but really have no clue what they are doing and just aren't cut out for this kind of thing. Windows is great for these people because it doesn't let you tinker with the internals. Ubuntu however, is based on Linux. Therefore, everything underneath can be customized using config files and what-not. These people can dig into all this and mess up vital config files like xorg.conf. That's not good.
Windows then does a steep decline, because more computer-savvy people tend to dislike Windows for being both a pain in the ass virus-wise, and a pain in the ass for getting in their way all the time and not allowing much customization. Ubuntu comes in here for having no viruses and allowing them to tinker with what they want. However, Ubuntu is a little bit of a pain if you want to modify your kernel or drivers or whatever, which is why Ubuntu drops off a bit at the end.