I like the control pixels give you, but I try to use em if possible.
It’s easy to get into a rights and wrongs of font sizing. For example, if a user has their font size set to be bigger and you drop a stylesheet that defines a font at 20px or 75%, then that’s changing the users requirement - so you *should* design for “100% of the users font size setting” I guess!
Ironically (in my experience) you hear a lot more about why you should/shouldn’t do it this way from accessibility zealots than you do from end users. In fact I don’t think I’ve ever heard an end user complain about not being able to rescale fonts, even back in the IE6 heydays (IE6 and its inability to scale pixel fonts is the foundation argument of why they shouldn’t be used)...but maybe one or two “that’s too small"s.
Yeah, I hear a lot of talk about it, but not from a user standpoint. Just the people that are the “standards advocates.” I’m all for standards, but I also like having full control across different browsers. Your average web surfer who knows enough to be able to open up Internet Explorer and do a Google search, in all honesty probably has no idea that you can even change a font size.