I've used two spaces for a while now. It just feels right that sentences have more space between them than "Mr." has after it.
But there's one annoying, persistent browser bug that cramps my style. If a line wraps after the first of two spaces, the second space will be on the next line, which looks terrible: http://i.imgur.com/MksV7.png
That's an edge-case of the way that person's software preserves multiple spaces by force in browsers; it replaces the second of two spaces between sentences in the source with a similar, non-breaking space. Use the `diff` command (if you're on a real OS) to compare; you'll see the two spaces are not actually duplicates of the same character code in the font used on that page.
But there's one annoying, persistent browser bug that cramps my style. If a line wraps after the first of two spaces, the second space will be on the next line, which looks terrible: http://i.imgur.com/MksV7.png