Been using them forever, only ever had one issue (the pic they sent me of a code had one of the characters cut off so I couldn't redeem it) and they gave me a new code within 20 minutes. Inventory is sparse though, I usually end up using a combo of CDkeys, greenmangaming, humblebundle, and occasionally fanatical. Deals is a good tool to use to find the lowest price on games but they don't list the grey market sites (cdkeys, c2a, etc) on there so you still might find cheaper elsewhere.
With CDkeys, my understanding is they exploit differences in regional pricing instead of gathering codes from questionable sources, which is why they are looked at as a step above the other grey market sites.
OP, if you haven't bought the game yet, consider using my referral code: God of War PC
I haven't yet but i'll use the referral code for sure.