To save space, I normally set Gnome's top panel to auto-hide.
But the bad defaults make it necessary that I have run gconf-editor to make the gnome-panel behave more responsive.

a) The hide/unhide animation is too slow.
=> Set "animation_speed" to "fast" by default.

b) When hidden, the panel still shows 6 pixel. This looks unprofessional when you have "expand" deactivated, because you can see a small bar of the wallpaper, which is confusing.
=> Set "auto_hide_size" from 6 to 1.

c) The hide/unhide delay is too large.
=> Reduce the "unhide_delay". (something between 0 and 100 should be ok.)
=> Reduce the "hide_delay". (something around 300 should be ok.)

d) Setting "auto_hide_size" to 0 does not work.
=> This should probably solved with a transparency.

Other information:
LP#387518