I guess I'll be the voice of dissent (and/or reason).
A high-cap 9mm will do the same thing, for less money, more options, and less headache.
The beauty of the 5.7, and the reason it was designed, was to use the same ammunition in both a handgun and a PDW, the P90. It needed to use lightweight ammunition that could penetrate kevlar. If your purpose is building a doomsday arsenal and availability of ammunition is not your concern, or you are protecting the president, maybe 5.7 makes sense. It is ballistically a great round, but its still a handgun round.
If your purpose is anything else, there are better choices. Shot placement still matters, so you still need to practice. I would much rather have a 20 round 9mm where I had some choices on features such as hammer/striker, safety, etc, and could know that I would be able to find ammunition in the foreseeable future. The 5.7 is a cool collection gun, but I think there are a lot of better choices as a "go-to" anything gun.