In reply to toddlahman.

A few reasons:

1) ReCAPTCHA has much larger range of applications, not limited to WP comments
2) It's not just relying on content but rather on user behaviour.
3) It's free.
4) Google is a more trusted source and they are much more likely to have tested their system more widely.

On a side note, speaking about spam, it is polite to state that you are the developer when praising your own plugin...