No at all - you just need to be careful what conclusions we draw.

We can safely report the statistic that "more people self report caffeine having an impact than ginseng."

The author goes as far as noting how many people are reporting an impact where there appears to be no known mechanism that impact could be occurring. That's a defensible statistic too.

Data from self-reported surveys isn't entirely useless, and is in medical research often enough for there to be studies on the kinds of error that arise from them: http://scholar.google.com.au/scholar?q=self+reporting+survey...