I really prefer to receive plain text. Even some CS teachers seem to think it's nice to set the text color to purple or blue, and I'm not even talking about spam...
That's why I like MailWasher, which parses the mail to plaintext even if it's not included. And that effectively kills e-mail worms too...

Unfortunately, the people that send me my cell phone invoice don't seem to get it. After sending about ten mails to the address specifically indicated in the message for complaining about the HTML, I finally get a reply... with the HTML invoice attached...