It was the Galaxy Note 3 in which we saw the first USB 3.0 cable connector, and surprisingly, it was the very first devices to have it. And while there are bucket load of new benefits of this new connector, I, myself find rarely using it (I only use it when I feel creative, or want to do something new and different). The biggest benefit of the new connector, is its ability to pull in 900 milliamps of power compared to the 500 milliamps of power that our old connectors used to pull.
Yes, it is a microUSB connector, but it is an updated version of it. Just like what Apple did with their new lightning connector. And to be honest here, I would say that the lightning connector is way better than USB 3.0, because it is not much of a hassle to use it.
It is definitely nasty and hideous, but it will definitely make you feel the difference when you use it.
Can you still use the old connector in these devices? Yes, but you will have to keep your eyes open while using the old one, because you might just damage the other part of your phone while trying to put the old connector in the slot of the new one.
And of course these new USB 3.0 connectors will only come in the flagship devices that you purchase.