Valve has posted the results of their user serveys on the Steam website.
These statistics show how long players took to complete Episode Two, average deaths per map, which advanced rendering options were enabled, and a whole lot more. They even have color-coded "topographical" maps that show where in the map most players died.
Unfortunately, there doesn’t appear to be any Portal-specific data.
Steam stats page: http://steampowered.com/v/index.php?area=stats
Episode Two gameplay stats: http://steampowered.com/status/ep2/ep2_stats.php
Results of Steam hardware survey: http://www.steampowered.com/status/survey.html