This forum is one of the most annoying places to try to log in. In the past, it's happened that even though I'd gotten my password correct on the first try, I'd still be told I'd "exceeded the maximum failed login attempts" and that I'd need to enter the Captcha. Then, entering the same password and the Captcha allowed me to log in.
This wouldn't be so bad if your captchas were so unreadable. Maybe I'm just unlucky with the monitors I've used, but it's really hard to read due to the colours used and the fact that some letters or numbers look so faded against a neon green or bright yellow background (and may end up being tilted).
I really wish you could use a different captcha system, at least.
Sorry for the rant, but I just spent a while trying to get the captchas right.