A couple of observations since getting a 57 Carbine. I think this is going to be one great home defense gun. Install a light, and a "shake awake" optic, and you have a pretty potent package. The ability for even a relatively new shooter to put multiple rounds on target in quick session, makes for a formidable package. The round also becomes less penetrative than a 9 mm bullet when it hits a sheet rock wall. So there is less chance a stray round will strike a loved one in another room. And lets face it, if an intruder is hammered by several rounds of 5.7x28, he will be less likely to continue to be a threat to you or your family. The U.S. Secret Service has used the 5.7x28 for many years in their FN P90 submachineguns, and have been very impressed with it. And as with ALL handgun calibers, it is still bullet placement that wins a gunfight.
There are very few standard factory magazines that hold more than the 20 rounds of the Ruger 57. And there are already aftermarket manufacturers who are making magazine extensions for the 57. I'm sure as this system takes off, there may even be a Ruger factory magazine with higher capacity coming. Even with "just" 20 rounds, I don't think a home owner is not going to have sufficient ammo to stop a home invasion.
The down side of this little carbine is of course the cost of the ammunition, it is higher than even most 5.56 ammo. I feel with all the new weapons chambered for the 5.7x28 cartridge coming out from Palmetto State Armory, CMMG, AR57, and this Ruger 57, the ammunition will have a much stronger market. The ammo companies can see the potential to stepping up the 5.7 production line.
So is the Ruger 57 perfect? No, but IMHO, with the addition of a light and optic, it is an awesome choice for a home defense weapon.