They're open to everyone provided they're willing to pay the £200+ price tag. Not unusual now to see 90% plus of the top end of an elite field wearing the shoes.
I run a lot of marathons but have zero interest in buying the shoes - partly because of the price and partly because it would feel like it was them making me faster, not my fitness / ability but then again, I'm not at the sharp end of the race, it's getting to the stage where if you're not wearing them you probably won't be able to be competitive at the front of the field