I think the site/service is pretty slick and well-made. It worked well for me -- I'd be curious to see what kinds of boxes these people failing were drawing (or whether it's just a browser issue for them or something).
These comments are all pretty negative, and I think the criticisms are mostly valid, but I don't think you've made a bad product (though it may need some tweaking, and captchas may be on their way out now for the reasons others have posted).
I just know that I've felt awful before when receiving similar comments to these others, and I would have liked someone to remind me: you made something pretty good, and it wasn't a stupid idea.