Can you elaborate a bit on how you drove customers to that site? I was looking at a similar idea just a few weeks ago but search engine competition on usability keywords is incredibly tough.
The only marketing I did was through my blog/Twitter. That spawned a fair amount of word of mouth and some other blog posts by people who had used the service. Beyond that I think it was just people talking to eachother because the product really does provide value if you're building a website.
I also have a hunch that the custom thank you videos played a part too but I can't prove it.