Hi Tissue,
The Flex Max from Slendertone will work. I used one, it'll give you a flat stomach, but won't get you anywhere near a 6 pack. You'll only do that by performing decent ab exercises. No pain no gain
If you use the Flex every day for 30 - 40 days, then you will notice a big difference. Need to have it for most of that period on the highter programs.
As for 'Ab crunchers' and similar machines, I am not keen. The reason being that it is easy to perform such exercises with poor technique.
You are better off doing a top drawer ab workout, including cable crunches and the like
Good Luck