The main attraction is models, which can be used as a UDA or in a static assert:

import concepts.models: models;

void checkFoo(T)()
{
    T t = T.init;
    t.foo();
}

enum isFoo(T) = is(typeof(checkFoo!T));

@models!(Foo, isFoo) //as a UDA
struct Foo
{
    void foo() {}
    static assert(models!(Foo, isFoo)); //as a static assert
}

// can still be used as a template constraint:
void useFoo(T)(auto ref T foo) if(isFoo!T) {

}

Here a template constraint isFoo is guaranteed to be true for type Foo. The difference between this and a regular static assert is that when the predicate fails the code for checkFoo is instantiated anyway so that the user knows why it failed to compile.

This library also implements its own versions of the range template constraints from Phobos so that they can be used with models:

import concepts: models, isInputRange;

@models!(Foo, isInputRange)
struct Foo {
    // ...
}