User interviews
User opinions are extremely valuable when assessing the more qualitative social acceptability of a product. However, the reliability of any quantitative data gathered is more questionable as it is the user opinions about the use of the product that are being gathered, rather than impartial observations. For example, users in general, and older users in particular, tend to either underestimate or overestimate their own functional abilities.
The quality of data gathered is only as good as the quality of the questions being asked.