I said it was ugly. But it's effective too. Your eyes, and monitor must have keen focus (or be running in native resolution for a LCD... I hate LCDs), to be able to see, and percieve the letters caused by antagonistic triggering of the of the neurons in your visual cortex.
I will repeat some of what was said long ago about this when it was just a mod for phpBB2...
This captcha works via an optical illusion. The letters aren't really there, they are generated in your mind by a wave phenomenon that causes neurons to get "tired", or shall I say, drained of charge. It is quite akin to this illusion from wikimedia.
The black dots aren't really there, they are being generated in your mind.
So yes, I could see why that would bother some visually impaired people. But it is the only way I could figure out with a static image, how to generate something robots couldn't see, and that is to use the faults of the human mind to my advantage.
Sure, most of the code isn't mine at all. It is after all the stock captcha with some tweaking, but the devil is in the approach.
My recommendation is to always provide an e-mail link of some sort for those who can't see the pattern. At least with phpBB, it's a one-time thing. And in consideration of that, shouldn't the admittance to the club be a bit tough?
Edit: P.S. As a bit of trivia, you can thank my Timex/Sinclair 1000 of long ago, where I first developed this technique after seeing something in a kalidescope pattern. Of course, you had to be across the room to see it with the huge quarter character pixels, but that's where this came from.