It's always a gamble. In my experience I have seen a completely clogged heads recovered 100% and partially clogged heads get worse after flushing/sonic cleaned. It all depends on how old the head is, what kind of ink you are using, how long the head has been clogged and just plain luck it seems. In some cases you might clear all of the nozzles but the head is still fried and will over-spray or have other defects. It's worth a shot when it's your last resort but it's not a sure thing by any means.