I needed to implement a capture quickly and used the invisible capture block.
It worked brilliantly, was very quick and easy to implement.  The demo is very useful showing how to use it as well as the documentation.
Others here at Kiandra have also used the "I am not a robot" checkbox and found it works great.

Thanks OSRecaptcha Team!

Only using the Checkbox so haven't tried the others, but for my use it works perfectly.