To be honest I do treat them as high priority.
Never really worked for a company where it's abused, 99% of the time an email is simply 'sent' and if someone has taken the time to mark it as High Priority I generally accept it is.
And as for Receipts, again - always: it enables me to keep track on who's read an email and action against it, likewise I send receipts back so people are aware that I've, well, er, y'know, read it rather than an email coming 5mins later saying "Jason, did you get my last email?"