Well I hope no one is expecting miracles. This is AMD. For years Intel shrunk and the clocks didn't improve. In fact they got worse. Most Sandy 2500k would do 4.8ghz. It was years before we saw another chip that would do that. They gradually got worse.
200mhz is a nice boost. IMO 4.2ghz puts them in the range of being perfect gaming CPUs. Sure Intel might be faster but you get more than enough FPS out of a cheaper CPU.
If you were having to buy a whole new board and etc for this? then yeah, 200mhz would sound far less appealing to me. But it's literally just a drop in upgrade. Remember, for years people did exactly that on Intel for 5%.