I have been doing alot of research on this topic as well, and here is what i have concluded so far.
ACR - Very good customer service. 24/7 phone and live chat. Fastest payouts (so i have heard). Software layout and speed is good. Although i find that bad beats happen alot more on ACR then any other site. for example AA vs KK or AA vs 33. AA loses alot of the time. I know its probability but theres a point when it becomes ridiculous.
Carbon - Good traffic. Great freerolls. Customer Service and payouts not to sure but i know there not as bas as Lock. Software can be kinda slow. you will enter a tourny and wont show up for a moment.
Lock - lets start with the good side of Lock. Best Traffic and best software. bad side. only email support which is slow, and the payouts are really slow. ive only heard bad news about locks payouts and customer service. They have said on twoplustwo that they are gonna change there payouts and customer service but that was months ago.
Please people feel free to comment on this and give their input. I would like to know more about these sites and other sites as well. I do not know much about rakeback or VIP programs.