Instapaper has great formatting, and repeated delivery works like a new issue of a periodical, but as far as I can tell, on a slightly unpredictable schedule (or at least on a schedule that is slightly inconvenient to me).
Later on Kindle sends the content pretty much instantly, which is much more useful since it means that provided I leave a few minutes to make sure it all gets processed, I can send some pages just before I leave work, and read them on the way home! The big downside is that your page ends up text-only; it doesn’t keep images in the same way that Instapaper does. Also, having it in the Chrome toolbar is very nice.
The solution for me is Kindlebility. It uses Readability to parse the page into a sensible layout (including multi-page articles), converts that to a PDF, then emails the result to your Kindle. It’s triggered by a bookmarklet, which means it can be added to any browser. Even better, it’s open source (more info in this blog post), which means that if I want to, I can avoid using any middleman at all!
Right now, this is by far the best solution I’ve come across so big thanks to Daniel Huckstep for putting it together!