I will refer to the excellent article written by TrevorXavier: http://www.reddit.com/r/Bitcoin/comments/1frm4x/provably_fair_by_bitzino_not_provable_with/

And provide a quote from it I also believe to be true.

Quote 1 “Bet Discrimination Under the assumption that an investigator will not place high stakes to investigate cheating, the house can safely offer a fair game to those using play money or very low stakes. As the bet size increases, so does the probability of an exploit. A house can also analyze betting patterns (progressives) for ways to minimize its short term risk of ruin.”

PrimeDice i believe is fair for micro bets.

PrimeDice i believe is unfair and cheating on bets 0.5 btc and above.

All should read the post linked. It simply is a must for anyone involved in btc gambling.