I do believe PDF is now an open, ISO standard. That said, unless it's a technical manual/textbook/something else I want to print, I don't want PDF for my ebooks. I tend to read on a variety of screen sizes and orientation and that usually requires conversion to various formats. While PDF is a good end-format (assuming a standard screen size to match the PDF), it doesn't really lend itself well to conversion unlike (X)HTML.
Someone cited the iPhone as a good PDF reader. For its size, yeah, it's pretty decent, but the PDF reading experience using the built-in reader leaves much to be desired. I don't really fancy having to pinch and flick all the time just to navigate a letter-size document.