But if you look at the video, it's clear they're a long way from making any kind of agile turret out of this. It's HUGE.
It is not that big compared to the guns on a destroyer or a battleship. It is still large and bulky, but not that far off.
Current destroyer's gun: http://en.wikipedia.org/wiki/5%22/54_caliber_Mark_45_gun
Iowa Class gun: http://en.wikipedia.org/wiki/16%22/50_caliber_Mark_7_gun
Yeah. The part of the turret you can see when you look at the ship is only the tip of the iceberg.
They'll be able to hit incoming missiles, but the primary strength of the system is as long-range artillery for land attack. It won't have the rate of fire you'd need to defend yourself against multiple incoming missiles.
With a missile coming toward you at Mach 2-3 you don't want to be shooting subsonic cannon shells at it.
The rail gun ammunition is much smaller so you can carry much more of it and make a much more mobile gun turret because it doesn't need to store tonnes of shells or have a feed back to a magazine.