Warlock is a nice build.
For grinding you will have a lot of skills and options on how to kill the mob.
Which makes it a fun build to play with.
Also its survivability is extremly high for a pure int.
If you play it well you can even tank GPs.
For Pvp:
Against Holy Spell (Thats the 76 cleric one

)
You are fcked. Just do as much DoTs as you can, and use your blood flower 215% skill.
and hope for the best, if not.. you are death.
*reflect might help as well*
Against non-holy spell:
If you lose.. you are a epic fail warlock.
If they other player has no holy spell you should ALWAYS win.
Disease first.
Stun / Sleep next.
Debuffs
Offering or any other 200%+ skill.
It should be enough to take out anything.