OK, I've now had a chance to try this out. For what it's worth, here is my feedback:
1. At first it didn't work, but that was my fault. I was confused about the free email delivery. I chose the email option, and entered the free version of my Kindle's email address:
XXXX@free.kindle.com. Nothing happened. Then I realised that you only want the bit before the "@". When I put that right, the file arrived OK. (It would have been nice to have seen an error message if the email address is invalid, but I guess there are good reasons why you can't do that.)
2. The actual formatting produced mixed results. I tried it with the main page from Wikipedia. All I could see was the "On this day" section and nothing else. And all the line breaks were lost, so the text was all merged into a single paragraph.
3. With the other pages I tried, the results were better. I tried a bunch of recipes, and and they all came through beautifully formatted, but without any pictures. I then tried some technical articles. Again, the formatting was preserved (including the fixed-pitch font used to show program code), and the pictures came through fine.
4. With all the pages I tried, non-ASCII characters were lost; they just appeared as an empty square. These characters included accented letters, fractions and currency symbols -- all pretty essential.
5. I agree with Ea that it's a nuisance that the files all have the same name (the browser inserts a sequence number to avoid clashes). I realise that the original page title is preserved in the meta data, but it would be nice if the filenames could also reflect the content in some way. That would make it much easier to manage the files on the PC before transferring them to the reading device.
Despite the above negative points, this is a useful service, and I hope you'll be able to develop it further. I'll certainly use it myself, and will keep an eye out for further developments.