I've got a midnight blue silicone case with Smart Cover on my iPad mini 4. My iPad has never been in a different case and I don't know if it ever will. It's super durable from what I can tell (I haven't dropped my iPad), and it doesn't really get dirty. It gives the iPad a much better grip and I never feel like it's slipping. I bet the leather case feels pretty great too, and it obviously looks way more classy, but I don't usually go for that look.
The Smart Cover is pretty great too, and the fact at you can take it off whenever you want is really convenient for gaming and some other stuff. It's also pretty durable and doesn't really get dirty. One minor gripe is that the microfibre on the inside does wear down over time, especially if you rub it on your fingers with everyday use. Of course, that's normal but it's still worth mentioning. I'll also add that you can't get a Smart Cover for iPhone but they're great for iPad.