GF lasts for 10 seconds. JF is a must have for almost any build. Pierce the veil is not a must have but I am going to keep it in my builds. That leaves 1 skill slot... not too crazy about a 10% intelligence boost for 10 seconds, I dunno.
I want you to convince me!
1. It stacks up to five times, giving you the potential to have 50% increased intellect and 50% regeneration of your maximum mana.
2. Health orbs are shared between teammates, therefore you don't have to go out of your way to collect health orbs in melee range if you have a barb/monk.
3. Some WD skill runes increase the drop rate of red orbs such as Death is Life (grasp of the dead) and Final Gift (zombie dogs)
4. As a couple other people said, intel does not only increase damage but also resistance.
You're not supposed to have five stacks of GF at all time, it'll probably be somewhat rare depending on your build that it gets stacked up that high. However, depending on the size of mobs you can expect to usually have at least one stack, probably averaging around two depending on your build.
GF increases your mana regen, damage and survability.