Text sizing in ems
I've frankly never encountered that many problems using the ems unit. I have people tell me that things were "too small," but that's usually because the user had their font settings set to "small" or "smaller". Silly people.
Regardless, this is one of the best explanations of how relative font-sizing works: How to size text using ems.
If the world were an ideal place, we’d all use pixels. But it’s not, we have the broken browser to contend with. IE/Win will not allow readers to resize text that has been sized in pixels. Like it or not, your readers will want to resize text at some point. Perhaps they are short-sighted, doing a presentation, using a ridiculously high resolution laptop or simply have tired eyes. So unless you know (not think) your audience won’t be using IE/Win or will never wish to resize their text then pixels are not yet a viable solution.
The page goes on to explain how to "start" with a particular fixed unit size (ie. px) by using percentages of the default value.
As a note, I'm not sure that I would ever make the body of a non-personal site start at 62.5% of the user's default.