FYI, most people are *massively* underestimating the value of spell penetration gear right now. According to the spell dmg formulas, as long as your target has any resistance left (and most raid mobs have quite a bit more than 75 resistance, so warlock curses aren't bringing them to zero) 4 spell penetration will reduce the avg amount of damage the target resists by 1%. Sign me up for +26dmg, +5% expected spell dmg.

Usually, warlocks have lots of resistances, thanks to demonology abilities.

This ring, together with Don Rodrigo, Rune of Perfection and Shackles of the Unscarred, can be a very good boost to opponent's resistance lowering.

There are some good items for this purpose in the game, don't underexistimate them, cuz they are really good in PvP.

No, you can't have negative resists anymore, there is no vulnerability bonus, the end.

It could be better, but with this, Don Rodrigo's Band from AV and the Rune of Perfection from WSG, you have -60 resistance straight away for the enemy, and you can get others, for example the Shackles of the Unscarred give -10, and the 5-Piece set bonus for the Arcanist set is -10 again, so if the enemy had anything less than 80 Reistance to your spell, they would have 0 :D, and I think this stacks with the Warlock's Curses, so -80 - 75 = -155 resistance, unfortunately though, it can't go into negative resistance... the +vulnerability bonuses for Warlocks then would be godly...