Recently I had to switch to a combination solution using Register Plus and Invisible Defender because I was running into several problems with Sabre. Note that I really like the concept of Sabre and would like to go back to using if the problems are fixed.
The problems I was having (seemingly out of the blue) was that either users that were added by the admin or users registering themselves were unable to complete their registrations due to errors. Also, when using the white background for the captcha, the captcha became invisible.
Please do some systematic testing of Sabre under WordPress 2.8.4 and fix all the problems that you find. I would love to go back. Thanks!