There are talks that Google's famous spam blocking service reCAPTCHA will soon be a premium service. Rumours are that the free version will cease to exist.
But looks like it is rumour after all. Premium is only applicable if you go over 10 lakh queries per month or 1,000 API calls per second. So if your project is small or mid range one, there is little reason to worry according to the news circulating right now.
Apart from the level of privacy that is offered under this service. I'd steer well clear and use a honeypot field instead that you control.
Google might actually make them paid. This could be testing the waters.
Then how can they collect all those info ? Paid services are not Google's strong suit. Not so far.
But I appreciate the service they are providing. reCAPTCHA is very efficient against bots.