What's with all the Expert Network companies?

Over the past year or so, I've gotten cold requests from a half-dozen or so different "Expert Network" companies, offering to pay me to have a 30-90 minute meeting with their client to answer questions about domain I have experience in. I accepted the first time, and was paid $200 rather promptly, but the whole thing feels weird and borderline scammy. I assume some of these companies are scams! Many are caught by gmail's spam filter.

Is this a recent proliferation, or did responding to the first one put me on a shared list? Have you had good or bad experiences with them? Are any of them particularly reputable?