The 390/390X are probably just cards to fill a void in AMD's product lineup and a hold over until a node change next year. I think the Fiji pro will be the card to lookout for - hopefully the pricing is not too outrageous.
It's also interesting to think about how more VRAM on the upper mid range parts actually makes sense - though we will never admit it - because the people buying the very high end cards dump them in 6 months for the next card, but the people buying the middle of the road cards are probably more value oriented and hold on to them for a generation or two, making the extra (8GB) of vram actually worthwhile. 8GB of HBM would be nice, but I don't think we're going to see it. I think the Fiji XT with 4GB of VRAM will be a fast high end GPU, but be a bit of a quirky product in a year or two from now if and when it starts to run into VRAM limitations.
In a year or two when NV releases its new 14nm GPU the 980Ti with its 6GB of VRAM(not to mention 12GBs of the titan X) could be even worse off and that is what I expect. Look at the Titan with its 6GB of ram. It was supposed to be future-proof but look at it now. Giving enough RAM is not enough to keep your GPU relevant, if you are not going to optimize your drivers then it is a waste. What happened to the performance of the Kepler is what is keeping me from buying the 980Ti. I think it's planned obsolesce and I can't support that.