decent quality oil and regular changes should be no need at all to use a engine flush.

if you do use a flush though bear in mind any crap that may be sticking to the sides of the block etc is all going to get loosened up and thus in a very short time will fill the oil filter full of shite so you'd be best to change the oil filter within a few days of running a flush through it.

personaly i wouldn't touch the stuff though