Yes, DirectX 12 is a significant upgrade and even improves current and older hardware - the first DirectX to do so.
It's also cross platform on Mobil and Tablets too, and It's Microsoft's plan to have DirectX THE development platform. OpenGL and the new SteamOS are out there and coming so it's a good plan for Microsoft to keep improving their API.
The advances seem pretty significant on older hardware (i.e. DirectX 11 cards) and more so if they are AMD cards with multiple CPU's.
As you say we have yet to see a native DirectX 12 game, but I would be shocked if we didn't see at least a 20% improvement on the Xbox One. Also, Microsoft will keep refining it, so it will keep getting better over the next few years.







