The gun is a little pricey, but the ammo isn't bad at all. About $18ish per box of 50. It's made by one MFG so the quality is top notch. I have a little over 10k rounds of it (for my ps90 also) and have never had a bad round. Balistically its a great round and accurate to boot. I can easily keep all my shots on paper at 50yds and 8 out of 10 at 100yds. Yes, I was talking yards. ;)
As for the gun, it is a keeper. In my opinion it will never win any accuracy contests, but you will hit what your aiming at.
I like the fact that it holds 20rnds in a standard mag and 30 with a small extension. No need for carrying an extra mag. It is definately a great gun and fun to shoot. I would highly suggest getting one. They run used around $800-$900 on gunbroker. I have seen them as low as $700(but that is rare).
My favorite thing is that it uses the same ammo as my ps90. That makes more sense than having two different calibers to fool with, you know, if the SHTF or the zombies come out ;)
Pros-Magazine capacity, quality built, lightweight, fairly accurate out to 50yds, very low recoil, not as loud as most.
Cons-size(not bad but longer than Glock30).
Go here for more info. About every questions that could be asked has been.
FN Five-seveN :: FN Forum.NET
My FiveSeven with 30rnd mag
The serpa Blackhawk holster works perfectly for me.