Pretty sure the only ones mining ZEC right now are the Tahiti and Pitcairn GPUs. Since if you got Tonga/Hawaii/Polaris you are most likely back to ETH/ETC mining.
I'm switching my Tongas back to eth, but my Hawaii is staying on ZEC. I get ~330sols with optiminer and ~28Mh/s with sgminer-gm.
My Tahiti and Pitcairn GPUs have been on exp for the last few weeks, since that gives me better returns than ZEC.
The other thing I don't like about ZEC is that the difficulty fluctuates +-15% day-to day, while eth (and even exp) is more steady.
Looking at the ZEC difficulty chart it seems the difficulty hasn't changed since Christmas.
The EXP difficulty is more volatile since whenever there is a pump it jumps quickly.