Flat trajectory is hard to use as a selling point with a pistol that is likely going to be used within 10 yards when it comes to self defense. The firearm itself doesn't lend itself to be a target pistol really. It holds the same amount of ammo that my CZ holds of 9mm.
The way I see it, the five seven is a light weight pistol that has light recoil. These two things are good for new shooters and when it comes to carrying it. Unfortunately it costs 900+ $, its big for its weight, the ammo is hard to get and expensive, Its trigger isn't what I would expect for the money, it can't match the performance of most 9mm hollow point rounds, etc.
Again if the five seven could match the penetration of most 9mm hollow points (13 to 14 inches in gel) and still tumble, it might be something. Unfortunately that would require more muzzle energy then what it has right now, likely by 150 to 200 ftlbs more. Considering to get to 12 inches a solid copper round had to be used which penetrated straight with no tumbling, and that the rounds that tumble penetrate to about 10.5 inches at most, its a ways away. Of course to match the penetration of a 9mm it would have more recoil and thus not have that as a selling feature.
I don't even believe in the FBis 12 inch penetration test as a absolute must have. However when I see even 380 rounds out of pocket pistols hitting 11+ while creating solid cavities, the 5.7 really seems anemic. The possibility of its light weight bullet hitting bone makes me question its performance in self defense. Relying on tumbling to provide all of the wounding characteristics is even more of a variable then relying on a hollow point expanding. Not to mention the entrance wound will not be conducive to bleeding out very fast which is something to take into consideration since not all people who are shot just drop.
