I've just wasted half an hour of my life and work time just to register!!! At least change the text requesting the captcha:
Please enter the six letters or digits that appear in the image opposite.As a non-native English speaker I was trying to enter captcha in opposite direction, e.g. 654321 instead of 123456
Also make the error more visible, as I was blaming captcha all the time my e-mail format was wrong... the error text should be near the erroneously filled field, not somewhere above the form, and it should be BOLD RED, or just another color, not the same as all the text on the page.