ES is godly, if used correctly, like blck said.

You can figure that by maxing ES and putting 16 points into Telekinesis, every point of damage you take is 1:1 ratio on Mana then. If you have 3k mana, you can roughly figure on having an extra 1950 HP from that (assuming 65% damage redirect from ES, too lazy to look up a real #).

If you're a Lightning build, you don't even need to max ES, as your +skill and +Lit skills should have you covered.