FYI, SharpDX Direct3D12 is being used for example in Xenko engine (see here) and afaik, It has been working well for them.
Just a little bit more context for the readers about what Promit implies by "used our code", as one may come to the simplified conclusion that SharpDX is a copy of SlimDX or it "robbed" their code:
When I started to developed the early genuine prototype behind SharpDX (AnyCPU assemblies + full code generation of the API from the DirectX SDK headers instead of handwriting as it was done in SlimDX, more info in this old blog post), I indeed used some of the infrastructure classes and samples developed by SlimDX to boostrap it (e.g things like DataStream, RenderForm, all mathematics classes...etc.).
I then proposed the concept to the SlimDX team but it didn't work well (they wanted to rebuild from the ground up my prototype, I wanted to release it asap and evolve it in smaller steps), so I decided to launch SharpDX in November 2010 (on google code at that time). I have to admit that in the rush of releasing the code + using a tool like ReSharper that was replacing automatically all the License headers in files in bulk mode + my not-enough awareness/carefulness of licensing attributions (my deep apologize for that), the copied code from SlimDX was not giving Licensing attributions.
Promit sent me an email in February 2011 to fix this situation, which I did promptly, as you can verify that files used from SlimDX have the proper licensing since then (for example DataStream.cs), I may have missed some files when I did it, but hope not...
So, to put in perspective, this "used our code" is a very small fraction, measured under < 1% to the total code base of SharpDX.