Digital accessibility experts are often asked about the usage of screen readers and assistive technologies. For example, one will often ask “What’s the most popular screen reader?” This is difficult (if not impossible) to determine technically, but also has privacy implications and other problems.
The following two surveys provide great data and are provided by very reputable organizations. Keep in mind though that the respondents were not controlled and the sample sizes are relatively low.
Unfortunately, several shiny HTML5 features (such as the video element), ended up implemented to different levels by browsers, without good accessibility, and without an interface that satisfied designers (a nearly impossible task). This includes the date input.
Recently on Twitter, there was some discussion about accessible date picker widgets. Jason Kiss (@JKiss) did some analysis and gave feedback.