The great thing about using online panel respondents for your survey is the ability to profile exactly the kinds of respondents that you want. The disadvantage of using survey panels is that there is a lot of garbage data that's included.
For example: People who speed through a survey, people who leave gibberish for open ended responses, people who just click on the same rating scale number all the way down the matrix and more.
Nico Peruzzi, PhD from Outsource Research Consulting, has an ingeniously simple and effective way to get the best of both from the survey panel experience. His latest article on Research Access gives you tips on how to identify bad respondents and exactly how to clean them up with nothing more than a simple spreadsheet function.