IMO, the 5.7 is a nifty little bugger. However, it has it's drawbacks and it's niche. The first being ammo price, availability etc... I don't know anyone personally on island that's shoots it, but the guys I've talked to online say that unless you have a constant supply of well priced 5.7 available to you, reloading is the best way to go.
Some people enjoy the idea of using the same caliber for their pistol and rifle, and in this aspect, the 5.7 gives much better performance than any pistol caliber weapon system I can think of. Of course, your choice of rifles are limited, and your choice of pistol is just the five--seven. I think the original FN P90's are the tits, an incredibly effective PDW. Even though we can't get those nifty 10" barrels, the civilian legal ones are still very handy and balanced rifles. I personally don't see the point of an AR chambered in 5.7, unless you are dedicated to using a Five-seven pistol and really want that commonality. The slight increase is magazine capacity and less recoil doesn't justify the payoff in terminal capabilities in my opinion.
Unfortunately, there are a lot of debates on the effectiveness of the civilian legal 5.7 rounds available, especially out of a pistol length barrel. Many people point out that you are only pushing a little past .22mag capabilities, while some of the diehard 5.7 guys claim that it is still an armor piercing beast. You may want to do your own independent research on this.
However, from what I've heard, the Five-seven pistol has found a market with trappers, and varmint hunters that carry it to dispatch of small-medium sized animals. They enjoy the better trajectory and smaller holes on pelts when using them on coyote and smaller sized game. That's not really an issue hear in Hawaii of course.