To be honest, the type of memory being used is pretty irrelevant. If anyone can honestly say that GDDR5 bandwidth is holding back current games then fair enough. But it is not even close to being too slow yet. Yes the bandwidth available on HBM is massive in theory, but smaller memory amounts still restrict performance more than double the memory at a slower speed.
The 1070 will still use GDDR5 memory, only the 1080 will get the GDDR5X memory. Both cards however will ship with 8GB of memory. However you look at it, its a big step forward with some serious figures behind them.
I for one can't wait for the reviews and benchmarks to be released.