95% 0f our work is stainless & these are much better, less gouging, faster & better finnish., mild steel i'm not sure of but any contractors i've had around use the hard discs,

It depends entirely on what your goal is. If you want to remove a lot of material, than a "regular" disc is hard to beat. If you want a nicer finish without using finishing discs than the flapper wheels are very nice for that. We use both, sometimes one and then the other. It really just depends on how you want the finished product to look. The flapper discs are at least twice the cost and they will not last as long for heavy grinding. I consider them light duty and more of a finishing tool. I hope this answers your questions satisfactorily.

Mike Sherman
Shermans Welding

We use flap disks quite a bit for radiussing edges on pieces for bridge work. They don't last as long as hard grinding disks but they do a better, smoother job so it takes less effort and time and we have less rework. QA inspection likes the results too. So overall we feel they are worth using for that.
They do not work as well for grinding copes and heavy grinding. The flap disks will do it but you'll go through a lot of them.
One other drawback, the guys get used to seeing smooth surfaces and start using flap disks where they aren't needed. It doesn't hurt the product but it eats up the budget if you don't watch it.
CHGuilford