Flex needles are actually single point needles that have a flexible shaft. so they have the benefit of having the points only being about 4 inches long like a circular needle and the shaft like a circular cord so you don't have the long straight needle shafts. But they aren't attached in the middle. They have a stopper at the end to keep the work from falling off.
they aren't bad but i think you might as well just get circs so you aren't duplicating efforts since the circ can be used for straight or circ knitting.