I have some warlocks @ 69 and I've been playing around with all of the different specs and I find demon seems to be the best. In Pre-WoLK nothing beat affliction ( 25x dots can smoke anything if it lives long enough for you to get them all going )
the fel guard has a nice stun affect and the Master Conjuror is awsome.
the fire stone gives almost 4-5% crit chance and last an hour now.
I have a 30% crit rating with just blues from quest rewards and a few bosses at or before 69.
my level 70 destructo lock only has a 22% crit rating and he's relatively geared out with pre-WoLK purples.