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I believe that the round will last. I had the same question about the long term viability of .357 sig when that round first came out and it has been very successful.
I have been carrying a Five-seveN daily for the last couple of months. They are in very high demand in my part of the state as pbwalker noted. It is large and can be difficult to conceal. I use a OWB holster and a 5.11 vest to cover. That is gonna get really warm about mid July, but there is not much I can do about it.
For me, the biggest advantage of the Five-seveN over all the other high capacity pistols I have tried is that it is a very soft shooter. It is one of the best handguns I have ever used for rapid fire drills. It has excellent accuracy as well. From what I can tell, that is the real advantage of this particular pistol. For me it is not about the 20 round capacity as I can get nearly the same in a number of 9mm pistols and even a few .45s. It is about unloading a lot of rounds very quickly and accurately. I simply cannot put .45 on target as quickly as this round.
I don't think it will catch on too well for most carry and self-defense applications. I am not even too sure about police applications. For military applications, I definitely see future for the pistol and the round. If I lived north of the border, I would go back to .45 and .357 as these are my favorite self-defense rounds but for now, it looks like I am an FN guy.