If you are talking about Omni Power Valves then this is what Vteckidd says:

Omnipower Valves
Great flow characteristics, great price, shatter like glass LOL. These are REALLY good valves, but your giving up some things when you run them. THe omnis are a 2 piece design and made of a metal that likes to shatter. If you ever overrev or get conatc, they will shatter the face of the valve off and take your bottom end with them. Not a bad valve but be carefull .