Online Panel are used in Marketing Research to collect data. There is a price to pay, but you do get sample of your description. One of the challenges of research is to get the sample of choice. And Online Panel provide that service at a price.
In my business school, we use Panels for surveys frequently. Our PhD students use them so often that we have a budget set aside specifically for panel recruitment companies. A panel survey is useful when you want a reasonable cross-section of the Internet-connected population, and ideal for some specialised groups, such as registered nurses or finance specialists for example.
We have found that response rates and data quality are no different than for other survey respondent recruitment methods.
After some resistance in the early years, our ethics committee now is very happy to accept research applications that use panel data because it adds extra distance between the researcher and the respondent, ensuring true anonymity for respondents.
Having said that, it can be costly to get the sample you need using a panel company. You have weigh that cost over the cost of trying to recruit respondents using other methods.