My dad says the current Hopper UI programming guide is sometimes hard to read, with the black letters on the somewhat-dark-greyish background (especially for programs airing during the current hour), so that's why I'm patiently awaiting the carbon UI, which has better contrast with the white letters on the very dark-grey background.
Other than that, I love the fact that Dish has channel logos in the guide, and also a screenshot of the program itself when you press the Info button. I think DirecTV still has the older text-based guide interface with their Genie program guide and DVR list, unless they recently updated it.
If the 4K Joey is being released in January, like I read in some other people's posts (not sure if this is true or not), then I'd guess that the carbon UI would have to be released on the same day, because the 4K model doesn't run on the current UI, and I'm assuming it fetches the guide/UI data from the host (Hopper), which would give the impression you can't have a H2K/HwS with the old UI connected to a new 4K Joey.
Someone correct me if I'm wrong or over-analyzing this whole thing...