I use carbon rims and I know someone else who has the same. My excuse to get them was that I couldnt find anything decent in 27.5/ 36 holes..shitty excuse but oh well.
As they come straight from the factory theyll drill you whatever you want. The Company is called nextie. Ive used them for around one year now and have had them on my old bike that got stolen. No complaints so far 
Can you feel the weight difference? Compared to a rim that weighs 600g probably yes, to a light aluminium one idk…
Ah yes if they are not on sale they are around 160$/ pp