I agree with the problems with spreadsheets but they are a decent guide. I don't want to get into the whole haste v crit discussion as you can find other threads on it. I can only repeat what has been written in Buzzkill's guides for raiding warlocks where he recommends using hybrid crit/damage gems (assuming you're hit-capped ofc). His advice comes from experience at the highest end of raiding rather than theorycrafting - not that there's necessarily anything wrong with theorycrafting.
I've actually found on the sheet that with my gear the hybrid crit/damage gems are equal to or just ahead of raw damage gems. The haste/damage gems are always ahead of others but remember that the sheet overrates the value of haste at low levels.
Buzz's guides:
http://nihilum.mousesports.com/en/article/18
http://nihilum.mousesports.com/en/ar..._for_warlocks/