kaled - 6:50 pm on Jan 18, 2011 (gmt 0)
I've never used em since the meaning is less than obvious.
I used pt originally since it allowed users to resize text in most browsers.
I now use px to ensure layouts have some chance of not breaking.
I haven't tested browsers for resizing for some time, however, the last time I did, the only one that worked as it should was Opera. By "as it should" I mean resizing all text however the size is defined, framesets, iframes, images, absolutely positioned items, etc. - this means that layouts continue to work properly.
If a user selects "large fonts" in Windows, i.e. 120dpi instead of 96dpi, fonts specified using pt should be displayed 25% larger but fonts specified in px should be displayed precisely as specified. If a page is free-flowing pt may be better for the user otherwise I would say use px but avoid small font sizes.