It's tempting to use these, but we haven't found them effective. One ad netted us 200 responses from interested people, but many did not match the qualifications we specified in the ad or responded after we'd finished the testing. These also pose problems. Even assuming that the recipients expect e-mail and don't mind the mild spamming, we've never had much luck. Once they reply, each recipient expects to be able to test and expects a response.
Scheduling can take four or five messages back and forth.
In early tests, we sometimes test surrogate users when the real users are difficult to find. This can help eliminate gross imperfections in a product that might irritate the real users and waste their time. Testing with surrogates is okay as long as you realize that they may bring biases that your actual audience would not have. For example, when we needed to test ER physicians, we first used medical students. When we tested subsequently with real physicians, we found that the students had helped us refine the design, but their lack of knowledge about real life did send us down the wrong track in one or two places.