Using psychophysiological techniques to measure user experience with entertainment technologies by Mandryk R, Inkpen K, Calvert T, Behaviour and Information Technology, Vol. 25, No. 2. (April 2006), pp. 141-158. This seems to be the new trend in 'user experience' analysis: using psychophysiological techniques to attest/validate specific applications/environments. Here is roughly, the point of the paper:

Current subjective methods of evaluating entertainment technology aren’t sufficiently robust. This paper describes two experiments designed to test the efficacy of physiological measures as evaluators of user experience with entertainment technologies. We found evidence that there is a different physiological response in the body when playing against a computer versus playing against a friend. These physiological results are mirrored in the subjective reports provided by the participants.

The authors base their claim on the fact that current methods are either costly (coding gesture, body language... then there are some inter-rater reliability) or too subjective ( user preference) and then advocates for taking psychophysiological techniques into account: Galvanic skin response, Cardiovascular measures, Respiratory measures, Electromyography, Emotions identifications.

Some of the results:

The methodological problems that we initially experienced in collecting and analyzing physiological data revealed a number of caveats for conducting this type of research. For example, great care must be taken to avoid stimuli that affect emotional responses, other than the stimuli being investigated. Although we took many precautions in Experiment One, such as the caffeine intake, sex, and age of the participants, there were still effects that we did not predict, such as the responses generated.