By and large, reCAPTCHA is the most widely used CAPTCHA challenge on the web at the moment and Google itself has taken many steps to make reCAPTCHA as accessible as possible (read Google's statement on reCAPTCHA's accessibility). Independent analyses (e.g. reCAPTCHA Accessibility reVISTED and The accessibility of Google’s No CAPTCHA) also conclude that Google's NoCAPTCHA reCAPTCHA challenge is one of the most accessible for mouse users, keyboard users and screen reader users. Our own tests confirm this.
However, there is one single reservation when it comes to the second challenge that reCAPTCHA renders if the user is deemed suspicious. Specifically, people who are deaf-blind cannot complete the second image-selection or audio challenge, as neither the visual component nor the audio component is accessible to them. For this reason, if you enable reCAPTCHA, you must provide an alternative method for users to be able to submit the information, such as by providing your department or service's contact information.