I was a bit suspicious when information began to surface that the new z-Enemy miner was about 10% faster than other miners.  I had been happy with Raven Miner which had been giving me an average hash rate of 150-160 MHz over the last week.

I downloaded z-Enemy and started running it 18 hours ago.  Over this time, my MHz has jumped to and held steady at 200-205 MHz (20-25% faster for me).  I'm not sure what causes one miner to be more efficient than another, but it is certain to me that my informal observations concerning the hash-rate of the two miners are noticeably different.