On the one hand, UFCS is nice syntactic sugar for those who hate free function interfaces (a group to which I do not belong). But it's more than that. It's also an easy way to extend the functionality of existing types, while maintaining the appearance that the new functionality actually belongs to the type.
Here's how it works. Given a free function that accepts at least one parameter, the function can be called on the first argument using dot notation as if it were a method of that type. Some code will make it clear.
void print(int i)
int i = 10;
Notice that it works on both variables and literals (see the output over on DPaste, where you can compile and run D code on line).
For a long time, I was rather ambivalent about UFCS. I didn't see the need. After all, I have no problem with free functions. Then I found a situation where it's a perfect fit.
I'm using SDL2 in one of the many projects I've managed to overload myself with. The SDL rendering interface has several methods accepting SDL_Rect objects as parameters. While implementing a simple GUI, I wanted to maintain bounds information using a rect object. But I also need functionality SDL_Rect doesn't provide out of the box, like routines to determine the intersection of two rects, or if a rect contains a point. And despite not having a beef with free functions, it really does make a difference in the appearance of code when you have a bunch of free function calls mixed in with object method calls. So I started implementing my own Rect type, giving it an opCast method to easily pass it anywhere an SDL_Rect is expected. Then I realized how silly that is when I've got UFCS.
So I scrapped my Rect struct and reimplemented the methods as free functions taking an SDL_Rect as the first parameter. And now I can do things like this.
SDL_Rect rect = SDL_Rect(0, 0, 100, 100);
if(rect.contains(10, 10)) ...
auto irect = rect.intersect(rect2);
And so on. I also had need of a Point type, which SDL doesn't have. But it was ugly mixing 'Poin't and 'SDL_Rect', so I aliased the SDL_ bit away and it's now just 'Rect'. With the combination of aliasing and UFCS, it's possible to hide implementation details without using a full-on wrapper to do so. Of course, it's not entirely hidden as the SDL_Rect is still directly accessible and you can still use the type by name. But it certainly can come in handy.