CaptchaTronix. I like the dashboard statistics. It shows the average solve time and number of threads currently using. Hence I know if I need to increase my threads or not. Ordering, upgrade or downgrading is quick and easy.
Really not sure. Occasionally I set CaptchaTronix as external service in CB and watch it solve recaptchas. It does solve a few, but until Sven add the following feature in SER, we're only guessing the solve rate,
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https://forum.gsa-online.de/discussion/11916/captcha-solving-service-success-rate-new-feature-request