I did spot this survey recently on Steam (Its collected automatically) if you scroll down far enough it will show you what OSes people are using for playing Steam games:

http://www.steampowered.com/status/survey.html

80precent using XP compared to 15percent using Vista.

Now if even the gamers who are the people most likely to like the Latest Thing TM are massively chosing XP over Vista then this really demonstrates that Vista is unliked.