Well, there are all sorts of (probably easy-to-fix) bugs with displaying html content with 5.1b1 (build 4325), but here is a nice feature. If you click in the displayed html, select all (cmd-A), and hit reply, you get a nice reply with the text of the html in a text-only email. Just what you would want.
What is lacking is a) when nothing is selected, for PM to act as if everything is selected (as it does for text emails) b) a way to select a sentence, paragraph, etc. (Although selection of text in web browsers has always been implemented poorly, at least on the Mac). A

